How do I find an 8 3/4 rear in the jy

Dana67Dart is correct, you basically need an A-Body car that came from the factory with a 4bbl engine. Although, I believe that starting in 1974 (could have been 1973) all the A-Body high performance cars, with the 360 4bbl, went to the 8/14" rear end, the 8 3/4 was no longer offered after 1974.