The strongest BOLT IN rearend for out 1st Gen Firebirds is the Chevy 12 bolt from a 67-69 Camaro, 68 - 71 Nova. The next best is the GM 8.5" 10 bolt from a 72-79 Nova, Ventura, or other GM X bodies. Just be careful from 76 to 79 as they also had a 7.5" which I would stay away from. When I put the 8.5" in my 69 Firebird, I had to shorten my driveshaft one inch, with the 12 bolt I did not have to shorten it, but had to change the rear U-Joint. You can also use the 8.2" rear from the same cars, but not sure if they are as strong as the BOP readend in our cars already, but with upgrades they can be made strong.
This from a Nova site I am on, the dimensions for the 68-79 are the same as for our 1st gens.It shows how to identify the different GM rearends except for the 12 bolt.
https://www.novaresource.org/axle.htm.
