When I had my 71 Chevelle, I bought a 1969 12 bolt leaf spring rear from a truck, It had a posi that I used and had put it in my non-posi 12 bolt housing. Of course I had some one set it up right and it never gave me any problems. At least check it for a posi and maybe scavenge that and ditch the housing. Just some food for thought!
12 bolt truck pinion is 1.438 diameter
CAr pinion is 1.625 diameter
10 bolt 7.5 pinion shaft is 1.438
10 bolt 8.5 pinion shaft is 1.625
Truck posi units have totally different carrier offsets and will be tricky to pick the correct one to use in a car. PLUS the truck posi or open carrier is overall narrower than the car unit and will not be supported as good in the bearing caps of the car housing. (they will actually hang off the carrier saddle some ...so this may not be so great to support for high horse applications
Truck rears were all leaf spring except some of the early 2 wheel drive which used long control arms and coil springs. So NO upper control arm mounts for a g-body rear.
truck and cars can interchange yokes and pinion seals...both use the same carrier bearings and outer pinion bearing. Inner pinion bearings are different because of the shaft size difference mentioned earlier. Rear cover is totally different too.
WHY is the car 12 bolt bigger than the truck? - the truck was designed in 1962 and only needed the larger ring gear for hauling and towing...The larger ring gears dissipate heat better. 1965 12 bolt car rears were introduced with more performance in mind and shock loads, so they beefed up the pinion for less ring and pinion deflection under manual trans shock loads.