I use car-part quite often, their interchange is a little spotty. I was burned by them on a few electronic parts. mechanical parts worked out just find on interchanges though. Thats a good sign though, I'll do some more searching and post my findings.
I was able to get 2 heads off a 2003 Buick Lesabre. The parts interchange at the yard says that it will fit 95-02 Firebird. I found a head-less Grand Prix at the yard, and placed one on it, and everything seemed to line up good. Now I can focus on tearing it down and upgrading the parts. Heres my plan:
1) Tear down the heads and send them to a shop to be cleaned.