Overview
The SL-C is a great way to build your own supercar. It offers extreme speed, an ability to customize things to a degree that is remarkable, and its a head turner on the road or at the track. It also has the advantage of being a clean sheet design which isn't limited by donor parts and its not a replica of anything. Its an SL-C.
But building any car isn't really like a Lego. And the SL-C is no exception. You must be determined, be able to overcome problems, and generally keep a cool head, because sometimes - as with any complex project- you will find yourself stuck for a while.
We've found that the number one contributor to satisfaction with the build process is not having a lot of experience in building or working on cars, or being in possession of a large garage with tons of tools. What really matters is having the right expectations. This is quickly followed by patience and determination.This page is an attempt to explain things in advance, so your expectations are in sync with reality.
We've organized it by topic, in no special order, and welcome feedback as to new topics to discuss, especially from builders who have finished or are building their SL-C.
Gelcoat finish- what does it mean?
The car comes with a gelcoat finish that is really quite good in the industry. If you decide to paint the car, this means you'll do comparatively little body work. This equates to savings of literally hundreds of hours of bodywork compared to other cars. We are not kidding- some other builders of other mid-engined "supercar" kits are reporting 500-1000 hours for bodywork and paint prep.
Of course, given the condition of the SL-C body right out of the mold, you'll have much less bodywork time even if you elect to paint the car. Most builders find that there is little to no body work required- just sanding to prep the car for paint.
So what does it look like when you get the car?
Well, the body will have small mold lines that you will need to carefully sand down until they are level with the rest of the body. This is normal, and not a defect. When done carefully, you wont be able to see these lines in the gel coat unless you get close to the car. Usually, you can completely remove them.
You may also end up polishing the gelcoat for a shinier finish as well. If the body doesn’t gleam when you get it- it isn’t supposed to. You need to either polish it, or prep for paint. From 10 feet, the car can usually be polished to look very good indeed. Many people have come up to the factory race car and remarked on the paint- but it is just polished gelcoat!
A painted body will always look better than a polished gelcoat, mostly due to the effort required to prepare the body for paint- any minor defects will be corrected, and the car can be painted to look just as good, or better, than a factory car.
Comment from one builder:
"I was planning on going with a gel coat finish. I spent a lot of time being extra careful and patching the gel coat as I went along. While the car looked good in gelcoat, I ended up having it painted. I sure wish I didn't spend the effort trying to keep the gel coat perfect."
Will the body have imperfections in the finish?
Yes. No fiberglass body is perfect from the mold, and occasionally small defects will be present. You might see an area where the gelcoat was missed, or where a chip was created in handling, or where there are one or more other small issues. These can all be fixed with a small bottle of gelcoat that can be applied to the affected area, and polished out until it is fixed. These are a normal part of any fiberglass body and don’t constitute a defect per se- just a normal part of manufacturing low-volume fiberglass bodies.
What are the advantages and disadvantages of polishing the gelcoat instead of painting? Just polishing the gelcoat and correcting any minor problems can be done in much less time than prep for paint, so the investment of time and materials is very low compared to paint. However, a properly painted surface will last longer than a gelcoat surface. Since there is no UV protection, eventually gelcoat finishes will fade, especially reds. They can sometimes be brought back to life after a few years in the sun, but there is a limit to how often that can be done. Some builders polish the gelcoat and then spray the car with a good automotive clearcoat. This protects the gelcoat from UV degradation, and is a less-expensive solution that a full paint job.
Painting the car allows bodywork to be done, and if you are adding features like fender vents, they can be blended in much more professionally with a painted solution compared to polishing gelcoat, as the edges can be covered, and painted as well as the rest of the body. If you have damaged the car body while working on it, the paint option allows you to repair that damage with normal body repair techniques and completely hide them under prep and paint. That's harder- and sometimes impossible- with just a polished gelcoat finish.
Who are the best candidates for just polishing gelcoat instead of painting? A racecar is a great candidate for polishing gelcoat. You'll save thousands in paint prep, and the car will look very good on track, and in the pits. Many racers just polish and go, as it is faster and cheaper on a race car where show car finishes are not expected or desired. On the other hand, if you want to compete in car shows, you need to paint the car- this is the only way to achieve perfect or close to perfect results. Some builders with street cars like the gelcoat finish because, in addition to the lower cost compared to paint, it is tougher and withstands chips from rocks better- and small damage that does occur can be fixed with a bottle of gelcoat and some sandpaper.
Generally, if you are building a track or race car, a polished gelcoat finish will be more than adequate. For a street car with aspirations to local car shows, you will likely be disappointed with a gelcoat finish as it will normally have small imperfections that will be apparent to the car show crowd, and will want to paint, dip or wrap your car.
Body Fitment
The SL-C has really good fit considering it is a low-volume fiberglass-bodied car. Right out of the mold, you can get pretty good fitment with just careful movement of the various body pieces. And with the optional interior tub, body fitment is made even easier.
But no car like this will have 2mm body gaps like certain new production cars. Production car manufacturers can maintain those tight gaps in part because the parts are steel, not fiberglass, and because those manufacturers have economies of scale that permit them to make precision steel press-molds that are extremely expensive.
If you require very tight body gaps like your new Mercedes S-class, fiberglass kit cars may not be realistic for you, as even if you do the body massaging necessary to get to those gaps, the bodies will move over time and lose some of the gap precision you put into the car in the beginning.
What is realistic? Most cars of this type have 3/16” gaps, and the SL-C can usually get to that or 1/8” gaps at best.
Fabrication and Welding
You'll need to have an exhaust system fabricated. And you'll find it useful to be able to make small brackets, etc to mount things. You should plan to acquire drills, taps, files, a jigsaw, a hacksaw, etc., to make such small things. For example, if your car has horns, you'll probably make brackets to mount them to the chassis.
But in general, you don't need to fabricate anything you can't make with hand tools you probably already have.
You also don't need a welder, know how to weld, know a friend who knows a guy, etc., again, except for the exhaust system.
However, some builders have done a fair amount of welding and fabrication on their cars. But that usually comes from a desire (and ability) to make things different. That's fine, but the take-away here is that you don't need to do any of that if you keep to a basic build.
Don't be intimidated by some builders who make a point of all the fabrication and welding they did- that's great for them, and you too, if you want to. But except for the exhaust system, there is no welding or fabrication needed that you can't do with normal hand tools (a jig saw, files, drills, taps, etc).
Delivery Dates
Most kits are delivered in 18-30 weeks or less. But sometimes, stuff happens and they don't. Some of the things that affect delivery dates are listed below:
Deviation from the standard. For example, when you ask the factory to make the car fit an engine they've never done before, sometimes that takes more time. Or when a new transaxle is specified- it usually looks simple, but the details usually take more time than expected. If you can't tolerate a delay for these reasons, don't deviate from the standard, or better yet, consider whether building a car is really a good idea.
Changes to the order. For example, when a buyer selects a gelcoat color, and then changes it mid stream, it sets back the entire manufacturing order. Expect a delay, and don't be upset if cars that were ordered after yours are delivered ahead of you- changes cause problems in the manufacturing process, and while Superlite is very flexible, they can't make miracles happen very often. This is true for any change, including wheels, tires, chassis or body options, etc.
Options choices that are not high volume. For example, if you choose the carbon interior tub, it's likely not in stock, and will have to be built in line with others. Carbon takes longer, so parts made of it will often extend the delivery time. This isn't to say there will always, or even often be a delay with the carbon tub- just that it's an example of parts that are relatively low volume, that take longer to produce than the fiberglass versions, and that may contribute to a delay.
Delays from suppliers. Wheels, fuel fillers, instruments and other parts not made directly by Superlite are often the source of delays. Some people say “Why doesn't Superlite just change vendors to those that can produce things on time?” Of course, if it were that easy, it would have been done long ago. For many reasons, it is almost impossible to keep stock of every part for every car at all times. Sometimes a vendor will only make a production run every 6 months. If the factory has an unexpected run of orders, and existing stock that was thought to be adequate is exhausted, the parts just won't be ready until that production run is finished, no matter how often you, or the factory calls. Many times there is only one vendor of a part, and if that vendor is out of stock, there is little the factory can do about it. Most of the time, cars now ship with no or virtually no backordered parts. But it does happen, and you must be able to tolerate that uncertainty. Eventually, they all come in, usually well before you actually need them. But parts shortages do happen, and you need to be prepared for the possibility of that. This is actually the biggest source of delayed deliveries, and something the factory is constantly working to make better. But it is likely to continue to be a challenge for the long term, as is the case for any small-volume manufacturer that has to rely on other suppliers for specialized parts.
Occasional unexpected circumstances. Last year the factory moved to a much bigger, more efficient building to accommodate increased volume. That one-time move caused some cars to miss their expected delivery dates. They all got delivered, but the move took longer than anyone expected, and that affected deliveries. The factory doesn't expect to have to move again for a long time, as it have lots of space now, but it's an example of something that took longer than expected and caused some delivery delays.
Recently, shipments have been delayed because of the effects of COVID 19 on production and logistics. This is a problem that all kit manufacturers (and, really, all manufacturing in general) face, and will likely have continuing effects for the mid-term.
Despite all this, most kits are actually delivered on time and very complete. The key here is to manage expectations: The factory does very well for the industry, but there are sometimes occasional delays, and every builder needs to be able to understand that and be able to deal with them when (and if) they arise. If you can, you will enjoy the build!
Communication
The factory is a busy place, and sometimes communications get lost. We've seen cases where a builder sends an email or calls about a missing part, or a change to an order, and doesn't hear back. Then they sit and let it fester until they explode. This isn't a Good Thing, and everyone's blood pressure can be reduced by an occasional gentle reminder call or email. If you truly are emailing constantly and don't hear back, check your spam filters and your junk mail folders first. If you don't find a response, call the factory direct and give them a gentle reminder. No one at the factory wants to ignore anyone, so give them a chance to fix any communication problems.
It's also true that you shouldn't be a pest. There have been builders who call the factory 5 or 10 times a day. That's excessive, and takes time away from other people who have an equal right to be heard. Don't be That Guy who is the pest. Gentle reminders, patience and good cheer are the best way to resolve any problems.
What are the differences in build between the SL-C and other similar-looking cars?
The visual differences are obvious, but new builders often want to understand the differences among several similar-looking cars in terms of the build process itself, not a discussion of which one is "better", etc. That's a reasonable question, and the answers may surprise you, as the cars have surprisingly different implications for the builder..
There are two other mid-engined, V8 powered cars to consider. One is the Ultima, and the other is the GTM. In many cases, the two other cars are similar, so we'll usually discuss them together, comparing them to the SL-C.
Chassis Design
The Ultima and the GTM have a tube-framed chassis of conventional design, using a combination of round and (mostly) square steel tube, onto which aluminum panels are riveted. This is the first fundamental difference in the build process- while there are literally thousands of rivets on an Ultima or GTM, the typical SL-C has none, or a handful used to attach lightweight brackets. Air-powered rivet tools are relatively inexpensive, but pulling thousands of rivets in any case is time-consuming. It is reputed to increase hand strength, though, if you are using a hand-operated puller. :)
You'll find that a lot of time is dedicated to affixing panels to the chassis with either the GTM or the Ultima. There is no comparable task for the SL-C.
Suspension
The Ultima uses welded suspension arms specific to the car, along with cast uprights. Most reports indicate that the car has adequate geometry when installed correctly.
Similarly, the SL-C has custom suspension arms, but are machined from aluminum billet, along with the uprights. When installed correctly, the suspension provides a good range of camber adjustments, and good caster. There's not much difference in these two cars from an assembly perspective in the suspension arena.
The GTM, however, was designed using the Corvette suspension (sort of- the rear suspension dispenses with the rear spring in favor of coilovers) from the donor car. The real difference in terms of assembly, however, is that the suspension is mounted in such a way that the front can't get adequate caster, and sometimes not enough camber adjustment. This is a well-known problem, and there are solutions. The most common one is to cut off the mounting tabs, make an extension to the chassis, and re-weld them back on the chassis in such a way that the geometry allows for more front end caster. Some people have tried various kinds of shims, or other solutions, but in the end, the tabs have to be moved somewhere to allow enough caster. Not making this change yields a very unstable car at speed (a sufficient amount of caster is necessary for stability, especially as speeds are increased).
Drivetrain Choices
The next difference between an Ultima and GTM compared to the SL-C is that the Ultima and GTM are designed, marketed and supported for a specific, and narrow range of engines and transaxles. This approach has pros and cons: from the perspective of support, documentation and development, it makes the Ultima and GTM cars more consistent in terms of drivetrain over a wide range of cars. This has the advantage of allowing the factory or the aftermarket to produce parts like headers, exhausts, axles, etc., that have a good chance of fitting most of the cars. The documentation can be more explicit, since the choices are so narrow. In the case of the Ultima, more comprehensive packs can be supplied, down to the washer level, as they are all the same. The GTM depends on donor parts, so the variability is somewhat greater (did you buy a wrecked C5 or C6? Is it a normal, Z51 or Z06?), and a couple of transaxles are supported, so like the SL-C, the kits aren't as comprehensive in terms of low-level hardware compared to the Ultima.
The main disadvantage is that you are limited to the drivetrain choices already made for you.
The non-SL-C builder can assemble the engine and transaxle package knowing that pretty much every other Ultima or GTM now being assembled has the same drivetrain. This makes things simpler to assemble as there is less complexity, and a wider understanding of what has to happen, and how it needs to happen. Parts are simpler, because there are fewer of them as choices are so restrained.
The SL-C, on the other hand, has such variability in drivetrains that producing, for example, headers, is not possible in a production environment, because the header maker can't assume things like transaxle size and location, or even that the car will have the same kind of engine.
On the other hand, that same benefit also has the disadvantage that different engine and transmission combinations are simply not supported by the factory, and if you want something different, you are completely on your own. That's why you see SL-Cs with drivetrains from GM, Ford, Porsche, Ferrari, Lamborghini, Audi, and many others, while the odd non-GM engine in an Ultima is very rare indeed, and generally frowned upon in the Ultima community as a path to reduced resale value (since it varies from the "factory build"), and why there are almost no known non LS-powered GTMs (since the engine bay in the GTM is designed specifically for the relatively small size of the GM LS engine).
In addition, the different body options for the SL-C (you get a choice of the "race" or the "street" tail, or the open LeMans body style) make for more complexity in terms of different parts for things like intakes. With the GTM and Ultima, you normally just go the factory way, and get what you get. That makes it simpler, if potentially less interesting, and possibly less performant. For example, the GTM picks up intake air in a space between two very hot exhaust pipes, making it more of a "hot air intake", which is obviously bad for performance. Where the SL-C offers more options, you have to choose, and implement the option you selected. That sometimes takes more thought and planning.
Chassis Material
Another difference is in materials used in the chassis. Normally, this wouldn't have much of an effect on the build itself, but in practice, there are some implications for the builder. It's easier and neater to work with small pieces of aluminum when you want to make brackets and the like, but aluminum as used in the SL-C chassis costs slightly more than the equivalent amount of steel in an Ultima. Also, a steel chassis will need to be protected, usually with powdercoating, after the car has been completely assembled. This means extra work to first assemble, then disassemble and take to the powdercoater, after which the assembly process is done again. The fact that a powdercoated chassis can be ordered doesn't really address the problem, as some damage will almost always occur, and typically there will be grinding on the steel chassis when electrical components are attached. Paint can be touched up, but powdercoat typically cannot. The SL-C chassis can be left unprotected (a thin, protective layer of aluminum oxide will appear), coated with a brush-on or wipe-on coating like Ballistoil, or wrapped in one of the popular gold foil coverings. The SL-C chassis can be so protected after assembly, saving time and money compared to re-powdercoating, or painting a bare chassis as on the Ultima or GTM.
On the other hand, while welding will typically not be needed on any of the cars, if you do decide to do welding on the chassis, you'll need to be able to use the GTAW or TIG process as originally performed on the chassis if you are working on the SL-C. This is normally more expensive if you are hiring, and AC TIG welders cost more if you are buying one. The steel on the Ultima and GTM can be welded with less-expensive machines and with less-skilled weldors.
Body Fit Considerations
This is an area where there is considerable variability in the three cars. The SL-C is universally thought to have the best fit and finish, and many examples exist where the body needed zero bodywork, and no paint. However, most SL-Cs have one or more small (or not so small) defects that would normally require a very minor amount of bodywork to fix, or can be fixed with a bottle of gelcoat and some fine sandpaper and polish.
The GTM is known to be the worst in terms of body fit, by large margin. According to some who have built many GTMs, an average one can take around 1000 hrs, just to get the bodywork correct. That number has several components, from preparing the surface for paint (it's typically not a class "A" finish), to making the doors fit (apparently the GTM doors have been and continue to be the biggest challenge by far in terms of fitment, requiring much time to get close). and fixing the seams (the seams where body panels are made in the mold are often mis-aligned, and sometimes filled with debris that must be ground out, then filled with a mixture of resin and glass to to regain most of the strength that was lost with the grinding). All of this takes tremendous time, either by the builder (assuming he has the skills and inclination) or a pro (which translates to significant costs). Because of the way the windows are supposed to seal (but don't), the typical GTM often has builder-fabricated door channels, which take considerable time and skill to fabricate.
The Ultima is normally in the middle, but closer to the SL-C in this area. The body panels are thinner than the SL-C (and usually, the GTM), but generally of good quality, though as a rule not as good as the SL-C right out of the mold. Nevertheless, some Ultimas are not painted, but just polished, as is possible with the SL-C.
Additionally, the GTM body is not symmetrical from side to side. Actually, most cars like these aren't perfectly symmetric, including the SL-C and Ultima. But the difference is such on the GTM that most builders find that they need to cut out one of the rear wheel openings, and re-fiberglass it in a few inches away in order to center the wheels in their openings. The main pro builder of GTMs does this as a matter of course on each car he builds. That adds additional time and/or cost to the build. A properly fit body on the SL-C or Ultima does not require changes to wheel arches.
It's fair to say that within a manufacturer line, there is variability on a body-to-body basis, but experience has shown that the distinctions above are generally quite accurate.
Paint
It's hard to separate needed bodywork from paint, as you can't get to paint unless and until the bodywork is done. But assuming the GTM body fit is done, and the seams are ground and repaired, there is the issue of paint.
Unlike the SL-C and typically, the Ultima, the GTM cannot be polished out- it has to be painted. And to apply paint, the body has to be prepped, which means that waves in the body need to be removed first. Depending on the pickiness of the owner, and to some small extent the color, a GTM will take much longer to prep for paint than the other two cars. This is mostly a reflection of the lower initial price, which drives body quality.
The SL-C and Ultima will typically require little prep for paint, though every body varies, so be prepared to look at yours critically to plan where it needs to be worked, if that is needed.
Donor Implications
The GTM differs from the SL-C and Ultima in that a large part of the finished car is supposed to come from a donor vehicle. This approach is part of the Factory FIve marketing model, and has served them well in producing kits that have a low entry price, which is possible partly because their kits simply don't include things the others do (like suspension, brakes, fuel tanks, steering racks, steering columns, wheels, and other things).
So what does that mean for the build process? Typically, you'll spend a lot of time and space tearing apart a donor car if you are building a GTM, cleaning and refurbishing parts from it, replacing parts that don't meet your standards, and then dealing with the carcass when you are done. The positive way to think about this is that in terms of absolute dollars, it is cheaper (if your time has no value, and you don't mind putting used parts into your new supercar), and better still, there is the possibility of selling off un-needed parts on eBay or CL to recoup even more of the costs. Some GTM builders enjoy the process of stripping a wrecked Corvette, and trying to extract as much money as possible from the carcass to put to the car in other places. Another option is to buy a pallet of used parts from junkyards that do that sort of thing, or just get a list of all new GM parts and order them. There are some spreadsheets floating around the web with lists of parts and their list prices, and most of them seem to indicate that new parts for a GTM will be in the range of $8-16,000, depending on which car parts are from (e.g., standard, Z51, Z06, etc).
Both the SL-C and the Ultima have the same basic approach, which is to provide all new parts, except for the drivetrain. New engines can easily be obtained for either car. Only the SL-C has new transaxles on offer from the factory, as the Ultima, which uses Porsche transaxles, must source them from the used market as they are normally not available new from Porsche (this is changing in the US, however, as Porsche now offers "remanufactured" G96 units which can essentially be considered as new, and often are).
Fuel Tanks
Yep, there are differences here in terms of assembly- who knew?
The Ultima is relatively conventional, but made somewhat more complex by using two tanks, one on each side of the car. Consequently, plumbing complexity is increased, making leaks somewhat more likely as the number of connections increases. Compared to the SL-C, it takes more time to do the tanks and related plumbing. All in all, it's not a terrible design, though, if you are willing to live with the complexity of multiple fuel level senders, and the filling problems common to multi-tank solutions. Going racing is complicated by the increased cost of twin bladders, more plumbing, crossovers, etc. But for the street, it's probably an acceptable compromise.
The GTM stays true to it's donor philosophy by re-using the plastic Corvette tanks. On paper, this sounds good, as OEM parts are usually well-engineered, and inexpensive. In practice, however, just enough has changed to make it an assembly problem. GTM builders have to deal with chronic sealing problems, non-standard connections (these two are probably related), problems filling the tanks quickly, as well as balance problems that make filling and measuring the amount of fuel in the tanks accurately. Some GTM builders have resorted to just using one tank to mitigate these problems, but that reduces an already low range; it does seem to make a lot of the other problems associated with twin tanks on the car go away, however- but can you live with an 8-gallon fuel tank? There is an aftermarket aluminum tank solution that replaces the stock plastic tanks with custom welded ones. These have better, more reliable connections, but still suffer from the balance/fill issues in some ways inherent to twin tanks. And, of course, these are relatively expensive, and aren't supported by the factory.
The SL-C uses a single welded tank, with all connections on one side to make plumbing as easy as possible, as well as minimizing the number of connections. It's easy to buy a bladder/tank combo that fits in the tank area if you are planning on racing the car (such bladders are required for most all sanctioned races). It's the easiest by far to assemble and go. Actually, the tank is already installed in the car from the factory- you just need to insulate if you want to, and make hold-down tabs. Add three plumbing lines and the level sensor and that part is done.
Electrical
Wiring is a task that intimidates many builders. And there are significant differences in the way the three cars handle the wiring issue.
The GTM is by far the most difficult, as it requires you to carefully pull out the harness from your wrecked donor car, and modify it to fit the functions of the new car. There is substantial splicing, cutting, and re-terminating needed to accomplish this. There is also the risk that the old harness may have been damaged in the wreck that produced the donor, or in subsequent modification. There is also the disadvantage of using "used" wire in your new car.
Most builders, especially electrical novices struggle with this, as the manual isn't exactly clear on how to perform every wiring task (it's hard for it to be, as the harness for even the C5 Corvette varied over the years, and with different options, and occasionally, different colors for the same function).
The Ultima is much simpler, relying on a custom harness (they call it a "loom", in charming Brit-speak) that is identical in design to how cars were wired in the 50 and 60s. It's relatively simple, and does the basics relatively easily. Anything fancier requires a decent electrical knowledge, and much more complexity. Because the Ultima lacks some of the functionality of the other two cars (in terms of interfacing with a steering column with integrated turn signals, for instance), bringing that car up to modern standards is much harder. Recently, the Ultima factory has started to support an expensive steering wheel option that includes the potential to have such switches on the wheel. This makes the Ultima look more car-like, compared to the old solution which just had a splined end sticking out of the flat dash. Of course, the same option is available for the other two cars as well.
The SL-C formerly came with the Infinitybox 3-cell system with a custom harness. This is no longer available from the factory, though it can be ordered through Infinitybox. The car now ships with a conventional harness designed for the car.
It's possible to add the InfinityBox system to either of the other cars, but the cost starts at about $1,600 for the base 3-cell system- and you'll have to come up with whatever harness mods are needed to the universal default harness that ships with the standard system. You may also need custom programming, which entails either sending your modules back to InfinityBox, or renting or buying a programmer so you can do it at your home. Because of the GTM's issues, the folks at InfinityBox have helped wire several of them, and in fact have a specific program for them (but using a "stock" InfinityBox harness), so adding the InfinityBox system to the GTM not only makes things simpler, but is a path that has already been plowed.
In summary: the GTM is extremely complex to wire, implements a used harness from a wrecked car and is poorly documented, but inherits some of the power and complexity of the donor car. The Ultima uses a old-school harness and fusebox that works OK for simple stuff, but is hard to expand, and it's basic system can't ever have the modern things. The SL-C now has a conventional harness as well.
The above discussion considers the wiring for the chassis only. Luckily, if you use the GMPP controller kit for the LS engines, the engine is really quite simple- you just need to get it fuel, coolant, and two connections from the chassis harness (one for switched ignition power, and one to the battery). Aftermarket engine controller with integrated harnesses (like the ones from Megasquirt) connect up in basically the same way. For engine wiring, the three cars are similar, assuming you are using one of the GMPP style kits. The Ultima is potentially easiest of all to wire for the engine, as there is an engine package in the aftermarket that has everything needed, including an Ultima-specific wiring harness, ECU tune, etc. This is slightly easier than the GMPP-style solutions, as it is designed explicitly for the Ultima, and therefore doesn't require, for example, the harness to be unwound or extended to make the throttle pedal connection as is normally the case with the SL-C. However, the cost for the Ultima package is quite high, and is usually bundled with an engine, which may or may not be what you wanted.
Exhaust
Because both the Ultima and GTM have a narrow choice of engines and body styles, it's possible for the factory to include or offer exhaust systems. Not so much for the SL-C- you'll have to roll your own in most cases. If you can weld, it's easy enough to do so, or you can take the car to an exhaust shop and have them do it.
Both the Ultima and the GTM also have some aftermarket options for headers and complete exhaust systems, from Kooks, and from Schwartz. Those SL-Cs that have headers, had them custom made. Likewise, the rest of the exhaust system on the SL-C needs to be fabricated.
On all three cars, there is an option to use the factory LS7 headers. These are inexpensive, available all day on eBay, good performing, and also fit tight against the engine for good clearance. Many SL-Cs use these, as did the National Championship-winning factory SL-C. On the GTM and Ultima, however, these would require more custom exhaust work (as far as we know now) compared to their standard offerings.
In summary, the SL-C will require the most work in terms of exhaust, compared to the Ultima and GTM, largely because whatever exhaust system work is needed on an SL-C won't have the potential solutions from either the factory or aftermarket sources. The use of LS7 exhaust manifolds makes it somewhat easier, but the rest of the system still typically needs to be done by the builder (with a welder) or by a custom exhaust shop.
Completness of the Kit
As with most projects, you'll find that you need a trip to the hardware store occasionally to get those 4 odd 1/4-20 stainless buttonhead bolts you need. So how complete, down to the nut and bolt level, are the three cars?
Of the three, the Ultima can be ordered and will typically ship with pretty much every nut and bolt needed, if you slavishly follow the build manual. Neither of the other two cars really offer that level of completeness. For new builders, that can be reassuring. For USA builders, this is even more critical, as the factory is a long way away, and parts orders after the initial order can be very expensive to ship from the UK, as well as taking much longer than either domestic source (Superlite or FFR). Of course, if you are building in the UK, the situation is reversed! Like all the cars, as soon as you decide to make changes, you'll find yourself making trips to your hardware supply, though.
The SL-C is next, with all of the big stuff you need, though with a caveat- you'll need to source some small hardware, battery terminals, any custom brackets you fabricate, etc. That's partly because the SL-C can be built so many ways, with so many drivetrains, that it would be wasteful to ship kits with, for example, the hardware for a specific fuel pump setup or even engine choice, compared to the Ultima, which has a very small set of supported engines. The SL-C comes with all the suspension hardware already installed.
The GTM comes up last, in the sense that the expectation is that you find stuff from the donor Corvette. If you are stripping a wrecked donor car, you should save the bucket of bolts, washers, nuts, etc that are the result of that process- they will come in handy, even as you'll probably need to obtain more for some purposes. If you buy a donor pallet (new or used parts from a dealer or junkyard), plan to acquire more hardware along the way. You probably don't want to put used fasteners on your car, anyway. Because the GTM requires the most fettling (as you would expect, given the wide disparity in initial cost), you'll find yourself making more small things compared to the other kits. So a slightly higher small materials cost isn't unexpected with the GTM.