8 1/4 is tough, 8 3/4 is tougher. 4-speeds with traction definitely will stress the rear more than automatics. You can install a B-body (1965 preferred) in a A-body very easily. I've done it. All I did was oval out the "eye" hole on the leaf mount. Worked perfect and looked better (notice how dusters give a "backwards" 3-wheeler look from the rear??). It will also give you the 4 1/2 inch bolt pattern.
