I've always had muncies in my pontiacs and my buddy ran a super T10 in his 65 goat, he always thought his ST 10 was stronger, maybe it was, but they all held up fine. I used to own a 1970 camaro with a built SBC and it had a saginaw 4 speed behind it, somewhat light duty compared to the previous ones, but I had no problems with it either. Currently my 66 GTO has a factory HD Dearborn 3 speed top loader in it, actually built by FORD for Pontiac, it's probably the strongest of the lot. If I were building your bird, and depending on what gears are in the rear, I'd go with a muncie M20 or M21, they're not hard to find used. If $$$ is an issue, the top loader 3 speed are cheap to buy and are a really stout piece. That roof may have some hidden surprises, if so pick up a clean one and put the whole thing on. I just did a search on Ebay, and found a firebird/camaro Dearborn 3 speed with the correct factory shifter. If it's a good unit, that'll be a pretty cheap way to get a manual in your car.