i'm tanking out my stock drum rear axle and replacing it with a stock disk rear axle, problem is i want to have posi too. what size is the diferental housing? i keep hearing about 8.5", 7 5/8", 7.5", 7.625? how do i konw which one is mine? if it helps the rear came from a boneyard, and was off an 83 z-28

 

On the passenger side axle tube there will be a number stamped depicting th axle code, ring gear, etc. find a friend at the local GM dealer parts guy and they can decipher the code. it may be covered in grime so you might have to clean/wip ome grease off the tube. the number is "usually" located at one end or the other on the tube and usually on the top, but don't quote me on that. Just match the ring gear sizes to your existing diff. and you should be good to go.