255/60-15's on 15 x 7 wheels. Wheels have 4-3/8" backspace and 3-5/8" 'frontspace', so the 7" wheels are 8" wide edge-edge. Stock rear.
235/60-15 on the front.
I did roll the fender lip a little, or actually bent up maybe 1/2" of the lip on the inside. I just used pliers or vice grips, bending maybe 10 degrees at a time, going back and forth along the length until the bend was over 75 degrees, then finished off with a hammer and dolly. It's not hard, just do a little at a time. I have 255's on my red 66 too. Tire clearance is tight on one side since axle isn't perfectly centered in the body. Tough to make more tire clearance in a GS unless you cut the stainless ww trim.