what posi unit size?
 

im looking for a posi to change out my open diff. and i need to know what size carrier? 7.5 or 8.5? right now the car has a 2.73 open diff on an 86 z-28. how to figure this out?

It's a 7.5. The 2.73 is a 2 series carrier. Keep that in mind if you are sticking with those gears. 3.23 and up use a 3 series carrier.

or 7.62, same as 7.5. gm just wanted to confuse us a little but more.

how hard will it be to chage my rear from open diff to a posi? is all the work in figuring out the shims? anything i should really know?

r&r for the carriers is easy and doesn't really involve much in way of shimming. swap ring gear to new carrier ,check back lash, check contact pattern and you're all set. the carreir shim total thickness shouldn't be changed, the arrangement of the shims can be to correct backlash.

in all seriousness ive never taken a rearend apart, so i have no idea what you just said..any links or sumptin for the beginer in rearend?

Here's a good how too. It's trucks but still GM rear ends. Lots of photos too.

http://www.fullsizechevy.com/fscu/axletech/