'76 D100 83/4 rear end good or bad?

Wonder what it would cost to have the housing narrowed down to A-body width? You may want to check the axles out that rear. They may be the ones people cut down to A-body width to get the large bolt pattern. Heck, the brakes are sought out too...
http://www.moparmusclemagazine.com/techarticles/drivetrain/mopp_0205_rear_axle_and_brake_swaps/