The 8 1/4 thread

Help please.
I have a 1972, 8 1/4 rear with 2.76 gears. It is an open rear and I would like to change it to a posi. What do I look for, I have the 3.55 gears for it in the garage. Thanks for the 8 1/4 thread I am sure lots of A-body owners can't afford an 8 3/4.