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

63" flange to flange? Better break out your tape measure and double check that. Not even trucks or Imperials had rear ends that wide. You may be thinking of an outside drum to drum measurement...
To cut an 8.25 housing, you must have a jig to hold the housing ends perfectly square- it uses a caged flat roller bearing that rides directly on the axle (no inner race,) so it MUST be square and on center, or you will eat bearings and axles.
Doubtful that the axles can be shortened, the shaft necks down in diameter immediately after the splines, which doesn't leave enough meat to cut new splines into. In addition, an 8.25 uses a C-clip for axle retention, meaning you'll have to cut a new groove in the splines of each axle at the appropriate length.
All this means you'd probably be better off having new axles made by someone like Moser Engineering, who could also cut the housing for you.
Truth be told, by the time you invest in all this, you'd be money ahead just finding the right rear for your A body.
If, perchance, it turns out you have a narrower rear than you thought, you may be able to get away with just using rear wheels with a deeper backspacing- people have been doing this with B body rears in their A bodies for years.