I have this 81 base firebird that is coming back from paint in the next month or so. It currently has a pretty anemic 76 400 Pontiac in it that is stock accept for a cam and headers. With the 273 gears that are in the car it's still pretty slow even with the four speed from the 77 trans am that I have in it. I did just put a new factory style gas tank in the car.
Instead of rebuilding the 400 Pontiac I was thinking that I could just leave the stock 4 speed in the car and run an adaptor to a junkyard 6.0. That way I could have some decent power and still run with my stock 273 gears while still being pretty streetable. As an added bonus from inside the car the stock shifter would be readily viewable so the car would look stock. I have a thing against automatics in muscle cars and I don't really want to spend the cash for a t56 if I can help it. Should I try to go that route is there a controller that is necessary to have on a t56?
The stock 4 speed should bolt right up to the LS motor. Just need a 621 bell housing or equivalent. Should be a number of write ups on here how to do it. You can even keep the manual clutch linkage if that is what you are into.
That being said an overdrive trans is well worth the cost. It will make the car faster, get better mileage and massively improve the ride at highway speeds. Probably one of the best overall upgrades you can make to the car.
I put a TKO behind an LS2 in my Datsun. Shifter is in the stock location and even uses a stock Datsun shift ****.
I did a 4 speed conversion in my car and it turned out great. Even the 5.3 has good power with a 3.31 rear and a 4 speed. Basically you need everything that a BBC would need plus ls7 pilot bearing and adapter for the z bar linkage for the clutch.
But which 4 speed are you using? A muncie is fine, and the 2.43 or 2.66 t10 is fine, but anything else will probably bust under the torque of a 6.0, especially with a 2.73 rear.
It will be better with the LS swap, but still feel down on power due to the gears. Even the new Silverados with a 5.3 and 3.08's do fine, but feel down on power when you drive the same truck with 3.42's....then add a 6000# trailer
BUT they are geared different at the trans that you are, so it is better.