I put a 12 bolt rear in and a few days later after a couple good burnouts I got no reverse,unless I rev it and then it goes,but its like something holding it.Its like its stuck in drive I cant even fell it go in reverse when I shift it?

 

Its likley the forward clutch is seized . Unfortunatly that means its time for a transmission or a transmission rebuild

 

See if it tries to go forward in neutral. If it does, it's most likely your forward clutches are "welded" to the steels. This is pretty common, especially if the aluminum forward piston cracks. This is the main reason we update the input piston set to steel pistons.

Frank
CPT

 

This seems like my problem. What parts are needed for the fix?

 

Atleast forward clutches and steels. I'd suugest stepping up to a steel input piston set in the drum. The set comes with a new over-run, forward, and 3-4 piston, to replace your aluminum one's. You'll also need a late spring pack that works with the steel over-run piston. This is beneficial for a couple reasons. One is to make it stronger, and second, the steel pistons have molded seals, which you'd want to replace anyway. There's a good chance the over-run clutches will be wasted, too. This may mean you'll need over-run frictions and steels, too. It is probablethat the pressure plates will be bad, also.
I'd strongly suggest you have this thing rebuilt. Fixing the forwards might fix the immediate problem, but I'm guessing you'll have other issues. These transmissions have many design flaws from GM. Thankfully, there are ways to fix them all...

Frank