You have a few things going against you. It's not a 3-series carrier, so parts are going to be more scarce. You might check with this place:
http://www.quickperformance.com/GM-7...Bolt_c_78.html
However, behind a 350 and in a heavy-ish Caprice, you may want to consider swapping to a later 8½" 10-bolt with a locker. They are more durable, and a lot more plentiful. Something from 93-96 B- or D-body might just fit right in.