for whatever this is worth, my buddy has a 72 duster his wife drives.
it has a B-body rear in it, unmodified. the springs had enough "give" that the rear didn't need the spring perches moved.
it has the 10" brakes, but putting the 11" brakes on would not increase the width, as the backing plates go inward the little difference there is.
he is running a 15x7 wheel with a 4" back space and 225/70 tires, and has enough clearance he could go possibly one size bigger using an 8" wheel. there is lots of spring clearance, as well as to the 1/4 panel outer edge.
the car sits at stock height , but has a slight rake due to the front tires being smaller at 205/60, but on the same 15x7 wheels with the same 4" back space.
if it were my car, i would space the rear springs back 1/4-3/8", raise the car a touch back there, and i'm pretty sure a 275/60 on a 8" wheel with a 4-4 1/2" back space would clear easily.