At an impasse with rear wheel/tire fitment

The 7.25 rear is likely to blame. They're actually narrower than an A-body 8 3/4", the 7.25 in my '74 Duster was narrower by about an inch. So yeah, a 15x8 with 4.5" backspace will fit an A body with an A-body 8 3/4" and the stock spring location. All of the axle charts/guides say that the A body 8 3/4 and 7.25 are the same width, but they're not.

I would install longer studs and use a spacer. I used Mr. Gasket #4311 studs on my stock 8 3/4" axles. http://www.summitracing.com/parts/mrg-4311

Maybe it's not an afternoon you want to spend pulling the axles and changing the studs, but its by far the least expensive way to go. And if you are planning on swapping an A-body 8 3/4" in later your rims and tires will be set up correctly for it.