The only thing I would add to the 8.5 Chevy conversation is that they are 1 inch narrower then the Pontiac units. I actually owed a 68 with a 8.5 from a nova and even though the wheels were only sucked in a half inch per side it wasn't a good look and you could really notice it. Of course that was running stock 14x6 rally wheels, if going with aftermarket wheels this is easily fixed by altering backspacing. I ended up changing the rear and when I measured to verify sure enough it was 1 inch narrower. Pontiac started doing this to there cars starting I think around 59 or 60 and marketed it under the name Widetrack. Also when using the Chevy rear they used a smaller u-joint but the aftermarket offers an adapter joint to solve this if you use a Pontiac shaft. And yes you are correct the number of bolts 10 or 12 is what holds the ring gear to the carrier but also happens to coincide with the number of cover bolts making it quick and easy to identify between the two with a quick peak under the car. oops I see I responded to bob s this was meant for Texas bird.