Starting in 73 Riv buckets are different, more like a fully split bench than the 66-72 Stratoback style. and even in the 71-72 Stratoback, backrest is much higher than a A-body
As an FYI, posi rears from a '71 Riv are pretty rare, especially with a 3.42, and worth some $$. If you have one, you can certainly find a buyer for it to fund your 8.5 rear.
Haven’t checked the gears yet Lifted the tires off the ground both spun same direction Just bought the Riviera GS it’s a numbers matching car
Post pics of the rear end cover. It will be either a 10 bolt cover 12 bolt ring Pontiac 8.875 rear or a 12 bolt cover with 12 bolt ring gear 9.3 both will have 31 spline bolt in axles. I HIGHLY doubt it will be the 8.5 10 bolt....BUT being the car is 40-50 years old...YOU never know. Jim JD