I believe you may be correct, except the engines are BMW, I am not positive about the ALMS cars. I was at Turner Motorsport last year on a tour, and its interesting, the Continental Tire GS and ST cars are mostly stock(ish) with modifications. The Rolex GT car (M6)that they were in the process of building at the time, is a tube frame, and other than the engine, and certain things like lights its pretty much all fabricated. Actually I believe all of the Rolex GT cars are tube frame except for the porsches, which will be that way in upcoming years.
If you cant see into the future you're not going fast enough.