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.
The biggest negative against this rear end is that it's currently a 5 x 4.25" bolt pattern, and the front is 5 x 4.5" (it has spindles, etc from a '73 with disc brakes). One option is to re-drill the axles, and there is someone local I found that can do it. It being a C-clip axle means there aren't any holes drilled in the stud plate, so the new bolt pattern can be drilled halfway between the old one. I do need to confirm the outside drum to drum measurement to make sure the wheels I want to run are available in whatever backspacing that turns out to be. And I would like to put discs on the rear.