I like the lean towards ALMS/GT3. It opens up a few good possiblities. But some of the cars would need to neutered or removed.
Cars like the Ford GT, Carrera GT, Vettes, Vipers, and Diablos fit more along the lines of a GT2 class. While the BMW, 911, F430, Panoz, TVR, and Lotus would fit better with GT3 specs.
If your considering opening up some of the late model muscle, some of them don't have the tire to compete. and would be default missiles just to get to the top of S class.
Maybe a split PI class like a 925 GT3 class and full S class for GT2.
And just to back up you and Hop, AWD is outlawed in both of those classes. If we could de-Quattro the Audi, I would love to give it a run. But include one AWD and then there will be complaining for Lancers and Subies.