[TA63]

Will the posi carrier and 3.08 gears from a 1980 turbo trans am bolt into a 77 grand prix?

[pippintook]

I assume your 77 Grand Prix has the Pontiac 400.
.
Not sure on interchangeability - but according to this site
https://eastcoastgearsupply.com/c-61...-8-5-inch.html
- both 73-77 Grand Prix and 71-81 Trans Am use the GM 10 Bolt 8.5"
- 73-77 Grand Prix and 71-81 Trans Am 8.5" 10 Bolts also appear to use 28 spline axles
Sounds like decent chance an 80 TA 3.08 gears and posi could work in 77 Grand Prix.
Guess we need an expert to verify

[fireprix]

Not all 73-77 GPs used the 8 1/2” 10 bolt. I’ve had 2 different 75 GPs. One had the 8 1/2” 10 bolt. My current one has the MP 78 sometimes referred to as the 8 3/4” even though the ring gear is 8 7/8”. I was told this is so back in the day it wasn’t confused with the more common 12 bolt. It’s the large scalloped cover with the drain plug. Gears will not interchange with the 8 1/2” rear. The MP 78 has a 10 bolt cover but actually has a 12 bolt carrier with 31 spline axles. Unfortunately the aftermarket doesn’t support this rear as it was mainly used in full size cars. I believe the easiest way to tell what you have is by rear brake drum size. The one I had with the 8 1/2” had 9 1/2” drums while the MP 78 has 11” drums.