Having owned an 84' Firebird and a 91' GTA, I would recommend a small block. A small block with a TES system from someone like Edelbrock would be my recommendation. You can get headers for a BBC in third generation F-bodies, but I doubt you can get off-the-shelf headers for a 455 Pontiac engine in your car.
A small block with the Edelbrock TES system fits very well. If your car came with a V6 in it, don't forget you will have to upgrade your front springs and radiator.
Third Gen. F-bodies are fairly cheap to build, hook real good with minor suspension modifications and are not too heavy. Good luck, Ed. www.edgesz28.com