Found the rules for the GT cars
http://grand-am.com/assets/GTRules.pdf
The gt3 cup cars as far as i can tell are the only cars that don't get much modification. The rx-8, which won btw, is a prep 2 car, which means it is semi-tube framed. It also uses a 20B 3 rotor engine instead of the 13B that comes in the stock car.