When I put a Chevy 12 bolt in mine I had to get a special U-joint. Spicer made one that goes from the Pontiac cross to the Chevy cross. Not sure it's still available, it's been awhile.
Wanting a Custom fit in an off the rack world.
I don't have time for a job, I just need the money.
Why not take your rear and have a posi carrier installed and new gears of your choice? Atleast you will know you have all new parts and no modifications will be needed
69 firebird 350 ho 4spd someday I'll get this thing on the road again