You'll have to figure out what bolt pattern you have. It should be 5x4" if it's original, and that will limit your wheel choices substantially. At this point though I'm not sure I would assume anything is stock if you don't know the history, so it could have been converted to BBP.
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What rear axle do you have? A-body rear axles were not all the same width, it depends on what axle is actually in there. The 7.25's are not the same width as the 8 3/4's, and that will change your backspacing needs.