michiganpat pretty much covered it, the difference between an A-body 8 3/4 with BBP axles and a 68-70 B body 8 3/4 is 1.15" per side.
So, on a Duster with 15" wheels and an A-body 8 3/4 and BBP axles you typically want to run a 15x8" with ~4.5" of backspace to get the largest wheel INSIDE the quarter. If you run those same numbers with a 68-70 B rear it moves the wheel out 1.15", which is basically the width of the stock quarter lip. So now your tires are hanging out of the quarter.
With SS springs it won't matter much for tire rubbing because your ride height means the tires shouldn't ever hit the quarter. But if for some reason you fully compressed the suspension, you'd put the tires into the quarters and it would go badly for you from there. Objectively, raising the back of the car ruins your caster setting, and it's hard enough to get enough caster on these cars already unless you have adjustable UCA's. The result being that without enough positive caster your car will handle like a shopping cart on meth.
In the front, you will not clear a 27" tall front tire. It will hit the corners of the fender opening, especially with a Duster the rear corner of the wheel opening gets to be an issue. The maximum tire height in the front is obviously ride height dependent, but anything over 26.5" is pretty much a no-go for most cars, and really anything over 26" starts to create issues even at the factory ride height. The 73+ brakes widen the track width, the 15" wheels limit the backspace, and both of those make it harder to run tires taller than 26" in the front with 15" wheels and the standard available backspaces
