All SL-Cs come with straight stainless steel pipes, 1.5" in diameter, with nicely formed ridges on each end to connect the radiator to the engine. So what's the problem? There isn't one, really, just a couple of ways to actually build the connection between the radiator in the front of the car, and the engine itself.
The Factory Way
The preferred solution from the factory perspective is to use silicone hose angled pieces to connect the provided pipes. This is fast, easy and simple. You'll need to get 45 and 90 degree connectors, which are available everywhere. Select the correct type of hose clamps, assemble, and call it done.
Here is a picture of stainless tubing using silicone connectors:
The High-End Way
Some builders who have access to welding equipment acquire pre-bent tubes and cut and weld them to form a single weldment that goes from the radiator to the engine compartment.
This is expensive in terms of time, but since there are fewer hose connections, there are fewer chances to leak. The factory can provide these pre-made for your car as an option, but because they are very labor-intensive, they are expensive. (see image to right)
If you do weld the tubes, use the GTAW or TIG process for the best results. You must back-purge the stainless tubes while welding or use a flux like Solar flux. You can use solar flux in lieu of back-purging, assuming you weld as you fit. If you don't know what back-purging or solar flux are, any competent TIG weldor will.
Note that custom-welded tubes typically can't be made from a set of angles and lengths, as very small differences in fabrication can make big differences in fit at the ends. You'll need to make these on the car, or have a fabricator make them on the car.
The Low-Buck Way
Some builders have a concern about silicone hoses and their reliability. For those builders, Joe Vask has devised a solution that uses off-the-shelf hoses available at any auto parts store. You need to use two of each part number; one is a one-piece solution to getting from the radiator to the long straight tube provided in the kit, and the other is used to cut up to make 45 and 90 degree pieces for connections. The long one-piece hose is 70885, and the part number for the hose to cut up into pieces is C71903, both Dayco numbers. The finished version is shown below:
Insulated 1.5" clamps from Aircraft Spruce here.
Blue silicon hose from Pegasus here.
Things to Avoid
Use stainless steel as provided in the kit, or aluminum if you are really on a weight-savings plan. Don't use plumbing pipe, or anything made of iron or mild steel- these will eventually corrode and have to be replaced. In some countries, notably the UK, some people like to use copper. This isn't bad, but stainless, as provided in the kit, is really the best option.
It should be obvious, but only use smooth transition tubing. And don't cheat the joints (by that we mean make all cuts and welds so that the finished part maintains the original radius perfectly). The picture below shows an example of cheated joints. Although the example is on a set of headers, the principle is the same.
The red lines show the joints that have been cheated. This set of headers is less efficient that it should be, because the transitions are not smooth and regular. Don't make this mistake with your cooling lines.
We've actually seen coolant pipes that were cut and angled, then welded to get a 90 degree angle. In addition to looking terrible, this approach causes increased resistance to flow, and cavitation, which have a host of problems. Don't be That Guy, who can't follow basic engineering principles.