I'm pretty sure the 305 and the 350 in the later year Firebirds were chevy engines. The POB blocks had very wide intakes and some had a oil fill tube sticking up in front. Without looking it up, I want to say 260-307-350-400-455 v-8s.
A true Pontiac v-8 can be a pain to install. They are wide, the starter is on the opposite side of a 2F/chevy v-8, bellhousings(other than a lakewood custom) have only the 4.686" diameter index ring. So, no sm465 unless you put a ranger O/d in there with the 4.686 nose and 5.125" tail. Limited to TH350/TH400 with BOP patterns for autos, and an sm420 with a car bellhousing.
Earl warden has done a few of these, and they came out nice, but if you are looking to be different, there are much better engine choice out there.
Keep it in the back. When you need to slow down, toss it out the back on a chain. You'll go slightly slower in that configuation than with it in the engine bay. Seriously, about the only one worse would be the 260 V-8 that I once had in a 76 Olds Gutless. Now a 455 POB block might be worth the trouble.