Could and the 5 litre pontiac gearbox be mated to a chevy 350.
With that question, are you asking if the 5 speed manual transmission used in the 3rd generation Firebirds with the 305 engine could be mounted behind a small block Chevy (SBC) 350? The answer is "yes", as the 305 engine in the Firebird was actually a SBC design (the GM "corporate" engine). In fact, the bellhousing would also bolt up. HOWEVER, you need to know that 5 speed gearbox wasn't the strongest in the world and that's why it was initially released for use only with the lower-output 305 engine.
If that's not your question, then of course that answer doesn't do you any good.