Widest Tire/Wheel combo for a '72 Duster?

I had 255/60-15s on the back of the Duster. I just changed them out for 275/50-15s. Got them on 15x8 rims with 4 1/2 back space.
Fit with about 3/4 to 1 in clearance by the spring. Didn't move
the springs at all.