I had a 79 TA, and here's what I know:
The 79 TA's came with a GM 400 with 4 speed manual, or an Olds 403 with TH 350 transmission.
There may be a change of driveshaft length if you swap, but I know for sure that the engine mounts are the three bolt clamshell type mounts. So if you're going with the newer motor in the first gen, you'll need the adapters. Classic has them, so does Year One, and I think Firebird Central has them as well. Maybe Summit as well. Do a google search, and you'll find them.

I'd want to know the block numbers for sure; that engine looks suspiciously like an Olds motor, but I could be wrong.

I just paid $1500 with shipping for a rebuilt 350 out of a 72 Firebird. It has 4k on the rebuild, and looks great. My 400 in my firebird is not original, and has a pretty stout leak in the crankshaft seal. Plus, the motor is from a b-body I believe, so it's pretty anemic. I'm thinking the 350 will have more oompf than the 400. But I'll tear the 400 down and rebuild her to around 450 hp.

Now, having said all that, for $500...I don't think you can go wrong. But you never know how the rebuild was, etc. Before I'd commit to anything, I'd want the block numbers for sure.