The Festival cars were the ones actually used at the Indy track by Pontiac employees, press people, Indy people, important people, etc.
PHS will tell you what build number your car is, whether it's a Festival car or not, things like that.
The rear end is a Borg Warner 7 3/4" (I believe) axle with a 9 bolt ring gear and is commonly known as a "9 bolt". It was also used in regular Trans Ams and Z28's, in the TTA it has a 3.27 axle ratio and is not as strong a 8.5" sadly. Putting slicks on it is up to you but I'd say prolonged dragstrip use will result in the differential going "boom". I'm going to put a set of Drag Radials on mine someday but I'll be gritting my teeth waiting for weird noises.
It appears to be really easy to put them in the 12's and not too tough to go 11's from what I read and what my friend Grumpy tells me.