I realise that I don't do well with what parts are what, but I didn't know that GM made an 8.5 rear. I thought that was a Ford deal.

As far as doing it correctly, GM 10 bolts, whether or not they are posi, are the weakest rears, compared to other rears of that era. A 12 bolt, or an 8.5 is a much better selection than the skimpy 10 blot rear.

I have a 3:23 12 bolt pois from a 71 Nova donor car in my 9. Sure, it's not correct, and some people know that 12 bolts are incorrect in f/g/f. Even so, it makes a nice statement because it belongs in the family, and compared to GM's 10 bolt, it's bulletproof.

If you have a drum to drum from a Nova, even if it's an 8.5, it will bolt up. I had to have my driveshaft shortened because the yoke or housing is about 1/4" longer than the 10 bolt.