As the Firebird is essentially the same as a Camaro, (1st generation F-Body) you should only need to locate engine and frame mounts for the Chevy engine.
One thing you need to consider is that the original transmission has what is referred to as the "BOP" bellhousing. BOP stands for Buick, Olds, Pontiac and is evidenced by the c shaped valley on top, Chevies have a peak on the top of the bellhousing. In other words, the original transmission will not mate to the chevy without an adaptor. You might be able to locate the adaptor at Jegs Racing. If the adaptor isn't available, you'll have to locate a Chevy specific tranny.
Also, keep in mind that all hoses, wires, and linkages will have to be adapted.
If you already have a good 350, you're probably working a good angle $ wise. However, if you need to procure an engine, be advised that the Pontiac 400 is available for a song. It wouldn't take much effort to find a decent engine in a salvage facility and go that way.
I purchased a "good" 455 last month for about what a decent 350 is worth.
I hope this helps.