70 and up Firebird or Camaro will not bolt right in. Get an 8.5" from a 72-74 Nova, Ventura or other similar body GM.
If you just want to add a posi just buy the posi carrier for the gears you want to run.
Also, I put in an 8.5" from a 72 Nova in mine a while back. You have to shorten the driveshaft by 1 inch.
When I went from the BOP to the 12 Bolt all I had to do was change the U-Joint on the rear of the driveshaft as the 12 Bolt yoke was a different size than the BOP one.