The windshield for the SL-C is an original design, and not from another car. It would have been easy to just use another car's windshield in the kit, and cheaper, but nothing from another car fit the body shape that the designers wanted.
There are two variants, a safety glass one that is the default, and is normally shipped with the kit, and an optional polycarbonate version for racing only (the glass windshield is DOT-legal, but the plastic one isn't).
The following steps discuss some of the issues for the glass windshield. The plastic racing one is usually held in place with several small screws and removal is trivial- just unscrew them all, and remove the windshield.
Fitting the Windshield
This is an area where many people struggle. Unless the windshield is dry-fitted before the rest of the body is aligned, it's likely to not fit correctly.
One builder wrote a great tutorial on how he made his fit perfectly here.
The take-away from this builder's experience is to make sure you fit the windshield first, before doing any door fitting, or body alignment. This is documented in the assembly manual, but few builders seem to do it in the correct order- always dry-fit the windshield before you do any body work, including alignment of the doors or front or rear clips.
Installing the windshield is easier than most people might think. It just requires care, the right tools, and another person.
The basic steps are outlined in the assembly manual, so they won't be repeated here. But generally, the keys are to:
1. Cut the windshield opening with a small enough flange, as one that is too big forces the glass to sit too high and make it impossible to fit properly.
2. Dry-fit the windshield before you do the bodywork.
3. Carefully apply the sealant on the glass while it is upside down just before you install the windshield. Don't leave any gaps, and don't over-apply it.
4. Tape off the area around the windshield opening to allow you to pull off any excess sealant. It's messy, and the longer you wait to clean it up, the worse it is.
Removing a windshield from a car can be done, but you need to be careful.
The main trick is to use a pick or heavy needle to carefully insert a thin wire or cord between the glass and the body opening. When you've done that, carefully sawing through the sealant is easy, if you take your time. Be ultra careful about the paint- you'll find that heavy tape will protect the painted surfaces as you saw through the sealant with wire.
Make sure you cut through ALL the sealant before you try to lift the glass off the body. If it resists, you probably missed some, and need to go back to be sure all of it is cut through.
Once the sealant is cut, you can carefully lift off the windshield with glass suction cups designed for this purpose (Amazon has them here, and other places have similar styles as well.) Don't press on the "wing" parts of the windshield, or you'll increase the chance of cracking it.