The 8 1/4 is in fact narrower than an 8 3/4. An A-body 8 3/4 with SBP axles and brakes is 57 1/8” at the wheel mounting surfaces, if BBP axles and brakes have been installed they're ~57 13/16”. For an 8 1/4 (or BBP 7 1/4) it's 56 3/4”.
So with an 8.25 you should have about 5 1/2" from the spring to the WMS. With a 4.25" backspace on a 15x8 and about 1" of section width overhanging the rim you'll end up with about a 1/4" to the spring, give or take depending on your cars tolerances and the particular tire you've got (not all 275's are the same size).
It's tight, but if it rubs it won't be by much and a small spacer would take care of it.
275's with the proper backspace will fit a Duster all day long and twice on Sunday. I tried 275's on my '74 Duster when it still had a 7.25 in it, worn out /6 leaf springs and the stock spring locations. It's tight all around but they do fit even with the car at stock ride height or lower.