Narrowing an 8 1/4" rear end, how is it done?

I’m guessing your GM 8.2 rear axle is probably from a Chevy II or 1st Gen Camaro. Unless it’s a complete hack job, I would probably just run it. It’s got 28 spline axles compared to the 8-1/4’s 27, and parts availability and aftermarket support is much better. Aftermarket custom axles (Dutchman makes really good C clip axles for a reasonable price) with Mopar bolt pattern and drum register / stub out length are an option, as well as disc brakes if you wanted.