C5R specs are on the "GTO" site:
http://gto-racing.nexen.net/gtworld_...orvetteC5R.htm (some text in French there but you'll find what you need)
The TransAm style cars are actually quite different, have a look at the regs:
http://www.grand-am.com/competition/2003_GTSrules.html
There is a list of approved engines and engine sizes, approved transmissions, approved "body styles" and body manufacturers, approved aero. Engines have to be approved beforehand also. Max. engine size is 6l (for carburetted engines). Live rear axle is mandatory, components have to be approved.
Besically, it is the reverse approach to an FIA/IMSA/ACO style homologation: you look in the rule book which specific parts can be used, then build the car (as opposed to building a car according to general outlines which is then homologated, and you have to stick to your own individual homologation).
The beauty of it all is that when, for whatever reason, you don't want to run a Chevy anymore you can swap the engine and body for a Ford and voilá, there's your Ford. Or, if the body you use (say, Olds) is not permitted anymore you can switch to a Vette body with ease. You might argue that these cars have a built-in identity crisis but there.