Where to start

98 Is a good year to start. The motors were pretty much the same up until 2002. In Camaros and Firebirds the LS6 block intermingled into a few of the bodies mid 2002. I would try to look for a car that is 2000 or newer. The main reason is not so much the engine as much as it is the electronics. It's much easier to get the software for 2000 and up to do whatever you want. 98's are a pain in that regard.

As far as performance, 11's on motor is easily done. Especially with a A4 trans. A simple head and cam setup with a converter will pretty much get you there without having to be too obnoxious. Hell, 10's on motor is also easily doable, but the cars start to get a little obnoxious. On spray you could easily mix it up with a good mid to low 11 second car and spray to get a low 10 pretty easily. There is a guy here locally that sprayed a 300 shot on the stock short block and went into the 9's with it. It certainly will not hold up to that repeatedly, but they can certainly be built to run the numbers. Especially with all the lower cost options that are now out there you should be able to do it on a nicer budget now than when we first started our crazy projects.

Anyway, I hope that's useful. Let me know if you need more info.