How about a small mod? By removing and relocating the emergency brake bracket on the trailing arm you increase clearance for 275 x60 x15 rear tires. It is a slight modification but really opens the wheel well.
I had 265 x 50 x15 on the front without modifications.
Without any mods at all, I's say the largest tire on stock wheels would be 255/60-15's front or rear.
I ran 275/60-15's on the rear of my 69, and 255/60-15's on the front with stock 15x8 rally wheels. I did a minor e-brake cable re-locate in the rear to prevent rubbing, and shortened the rear spring bolts by 1/2 inch.