You could probably get a b-body or c-body 8 3/4 rear cheap and have it cut down to your measurements. Then sell the a-body rear for what you have into the modified rear.
On my D/Dart, I have M/T ET Drags that are 28.0/9.0-15's mounted on 15x6 rims. The rims might have a 3.5 bs (measured...