Rear end

what kind of driving do you like to do? run on the freeway and keep up with traffic? run hard from 0-60? get an idea of what you want your cruise speed RPM with a calculator like this: https://www.tremec.com/calculadora.php

keep in mind, this will estimate cruise RPM probably 100-150RPM low, because this calculator does not take converter slip into play. try figure out your current ratio as well, to determine what ratio you may want to go to.

IF you decide to replace the rear, you can probably sell your current one for a decent amount, I'd be interested if it was in indiana, michigan, or ohio....don't know what the A body parts situation is down there, but up here, you can't find much. your best bet for upgrading the rear is to look towards a ford 8.8 out of a ranger or explorer, especially if you want to move to a 3.55-3.73 ratio, disc brakes, etc....a lot of them are sitting in the junkyards with limited slips, too. an A body 8.25" is about 57" or so drum face to drum face. a ranger rear is 58.5" mount face to mount face, an explorer is 59.625". the ford axles have the pumpkin offset, if you shorten one side to center it and use the same side axle shaft on both sides, it ends up being 56.625".

XJ 8.25's are an option, too. autos are mainly 3.55's, manuals 3.07, might find a tow package with a 3.73, might find with a limited slip....only problem is they're 60"-60.5" drum face to drum face, which means you'll need to use high offset (like +30-45mm later model mustang) wheels to not have the wheels hang outboard of the fenders.