[67drake]

I was in the same boat. I picked up a 71-72' Buick/Olds 8.5,but it had a 2.56 posi in it. I set it up with 3.42 gears and a ring gear spacer behind the ring gear,so that I could use the 2 series posi carrier with the 3.42 gears.
That rear has been sitting in my garage for 2 years now because I decided NOT to take the chance of using the spacer behind the ring gear. Too risky with the way I drive my GTO. I picked up a 3 series Eaton posi just yesterday,so I'll be redoing it all in the next couple weeks.
Those 8.5 rears are supposed to be stronger units then the 8.2's that came in our cars originally,so you will have a strong set up when your done (If you DON'T use a spacer!). I've heard if you put in a 30 spline carrier and axles,as opposed to the 28 spline that came in them stock,they are as strong as a 12 bolt rear.
As B-man said,the 8.5 carrier and gears from a Firebird will swap over,just not the housing. Also the carriers/gears out of many GM 8.5 rears from later years will fit in the early 70's housings. I forget the cut off,but somewhere in the late 80's as I remember.