With a Duster you can run 275/60/15’s on 8” wide rims with the leaf springs in their stock locations. The 65-67 B body 8 3/4 just changes the backspace that you need. I run a 68-70 B body 8 3/4 in my ‘74 Duster. It’s 60 1/8” wide drum to drum, compared to the 59 1/2” drum to drum for the 65-67. So, 5/8” wider overall, or 5/16” (8mm) per side.
Now, I have a 1/2” spring offset and another 1/2” cut out of the quarter lips, so I can’t just knock 5/16” off the 7” of backspace I run on my 18x10’s.
But through the magic of offset (instead of backspace), and the fact that I know my wheels are almost perfectly centered in the wheel tubs (they have to be with 295’s), it’s easy to calculate. I run a 38mm offset on my 18x10’s. The 65-67 B rear is 8mm per side narrower, so that’s 30mm. Offset is a centerline measurement so it isn’t effected by changes in width. So, a 15x8 with 30mm of offset will be centered. That’s 5.7” of backspace.
So, 15x8’s with 5.7” of backspace (or a 30mm offset) should let you run 275’s. That’s down to body tolerances. These cars aren’t perfect and they aren’t all the same, so you have to measure your car. I’d be looking for a 15x8 with 5.75” or more of backspacing, then use spacers to correct. Or run 255’s, which should give you a bigger range. 255’s should work with a 15x8 and 5.5” to 6” of backspace. Give or take depending on your cars tolerances. And all of that depends on you having moved the b-body perches to A-body specs.
