Rear end options.

Admit that you are in over your skill level, and get a professional to rebuild the 8 1/4. Any swap you are considering is far more effort and expense than you imagine. Besides the obvious needs of the proper width overall and for perches, you will need another driveshaft and some devilish fabrication of a parking-brake mechanism compatible with a different axle. Plus a proportioning valve to get brakes approximately balanced front-rear. The price of another axle and diff (and brakes) will well exceed any repair bill on the 8 1/4, and your work is just beginning.