Would a v8 trans am engine fit in a v6 firebird?
yes, you just need to get new V8 engine mounts welded on the frame and have the old V6 ones cut out.
you will also need a new fan shrowd and if its going form a chevy v6 to a pontiac v8 you will need a new wire harness new battry tray.
A 350 will easily fit but you could install a larger engine with major modifications. Stick with a 350 which was the optional engine in Firebirds. You will have to change the ECU also, so a donor car would be the easiest. If I were in your shoes, I would sell this Firebird and purchase one with a 350 already installed. You would be money ahead in the long run.
You can make it fit but not without some modification. You see the V6 Fbody has an angled throttle body and intake, Trans Ams have a different style connection. I have seen it done before but with all the trouble you will go through I would not recommend it. A Cold Air Intake would be a suitable alternative.