B body rear axle in an A body
The good news is if you have the large bolt brakes in the front, you could basically have "square" and rotatable tires.
Exactly!
That was my original motivation for the ‘68-70 B rear. I got greedy with the rear tires and so I ended up with a staggered set up I can’t rotate. But my offsets are basically the same front and rear, +35 front and +38 rear.
But the math works, with a ‘68-70 B rear in a Duster/Demon/Dart Sport and a 1/2” spring offset (you have to move the perches anyway so why not) you basically run a +30 to +35 offset front and rear and can rotate the wheels if you run a square set up.
If you wanted to stick with 17’s that would get you up to 17x8’s with 255/45/17’s with like a 5.7” backspace (+30 offset). The reason for that is the outer tie rod interference, with most 17” rims 5.7” is about the backspace limit before the tie rod becomes an issue. That kinda rules out 275’s with 17’s unless you get into doing more body work.
With 18’s the outer tie rod fits inside the lip of the rim, so running square you could do 18x9’s or 18x9.5’s with a +30 or +35 with up to 275/35/18’s. That’s a 6.1” to 6.4” backspace, for 275’s the minimum backspace is about 6” unless you really roll the fenders.
Obviously that’s a guideline, that’s the offset you need with 73+ BBP disks. Changing the braking set up can change the track width. And the body tolerances on these cars vary, most should be able to run those recommendations without too much trimming or rolling of fender lips but it all depends on the particular car and it’s ride height.
