I can answer your questions regarding carbs for your Firebird. I just completed the rebuild of our firebirds engine and I spent $600 on the dyno testing different combos. Everyone here will offer advise, very few have any DIRECT actual experience with their opinions they offer, as well as they are trying to be. Hardly any have dyno numbers and live testing to back up their recommendations. I do. It cost me some bucks, and I don't mind sharing.

I saw you were advised to run a RPM manifold instead of the stock iron manifold. I ran test on an engine dyno with a Pontiac 455. I only changed the intake, using the same carb, same day, same gas, same everything. The gasket matched RPM makes 3 hp and 3 ftlbs tq more than a gasket matched stock iron 1972 intake averaged from 2600rpm to 5600rpm. It won't clear the factory firebird hood without a dropped air cleaner, which will likely cost you more than 3 hp. So toss the performer, performer plus, torker 1 & 2 manifolds away if you are under 500HP.

I tested a 1968 RA2 Quadrajet (750) that had the shafts bushed, a complete rebuild with a Cliffs performance kit, and had the air valve, jets and rods optimized to allow more fuel. The carb was optimized using Cliffs book, and it ran great. The carb ran perfectly on my car, I had driven it about 600 miles after the rebuild with no issues. In fact, I though it was greatly improved over stock. If you recalibrate the idle bleeds, jetting, rods, align the blades for full throttle, and do all of the tricks in Cliff Ruggles book, it will run fantastic. However,...

The dyno test on the stock manifold had the Rochester at 394.63 hp. A 4165 Holley 800 CFM carb (mod 6212), out of the box stock in every way, made 490.3 hp on the same factory intake 2 minutes after the other run with nothing but the carb changed. I was shocked, as was the dyno operator.

The factory intake is a spread bore design. Use a spread bore carb (4165), as the adapter to a square bore carb has been known to cost 15hp.

That dyno taught us some amazing things. Some are thinking, well that wasn't fair, the Quadrajet is rated for 750cfm, and the Holley 800 cfm. While the engine was making 490.3 hp, it was consuming 605cfm of air per the dyno hat.