I have question about the 1966 grand prix. I am trying to put a posi and 3.73 gears in my 66 GP. I bought 2 different companies 8.2 ring gear 10 bolt posi units and gears. the shop doing the work on my GP said it doesn't fit. so we thought maybe the rear end had been changed out. so I found a rear end from a 65 catalina in a junk yard and pulled it out myself.

this is what i'm finding out after taking the 65 rear end apart and what the shop is telling me is under my 66 GP.

the cover looks like a Pontiac 10 bolt with the scallops on each side. the ring gear has 12 bolts and I think is 8 7/8 (still taking the 65 rear end completely apart). the axles have 31 splines and bolt in (I have one axle out)

so what I am wanting to know is what rear end did Pontiac put under the b-bodies and does any after market company make a posi and 3.73 gears for it.

any information would be appreciated thank you.