I suggest that if you want to stay with the OHC Pontiac engine (and you should!) that you put a 200-4r behind it instead of the ST-300 (Powerglide). The OHC engine works very well and isn't seen very much and usually gets more attention than a built V-8 at a show & shine.

An easy improvement is to bolt on the Sprint intake and exhaust manifolds to get it back to Sprint status. You'll be very happy with the combination of power and economy.

The engine mounts are the same for all years of Firebird, so if you have them you're good.