Rear end swap to an 8 3/4

An A body 8 3/4 would be your best option, BUT you can use an early B body rear with some work. B rears are somewhat less expensive and easier to find. You would have to use deeper backspacing on the wheels.

Here is some reading for you:

http://www.moparmusclemagazine.com/techarticles/drivetrain/mopp_0404_8_3_4_rear_axle_housing_swap/

I think Scott AKA pettybludart had an 8 3/4 housing for sale.