[SNAKEPILOT]

I am running 17x9.5s with 6.0 bs and a 275 tire on my 1967. I smoothed out a bulge that was on the inside fenderwell near the front and rolled the fenders. I have approx 3/4" clearance on both sides of the tires now. An extra half inch would fit but the body probably flexes more than a 1/4" under aggressive cornering.
Variables: Different brake set-ups affect offset/backspace.
The section width varies among tire manufacturers as much as .5 inch (translates to 1/4" per side).
Stiffer leaf spring bushings will minimize body flex (I have Global West Delalum = no flex).

With the rear end jacked up under the axle (both wheels off the ground) tape a ruler to the top of your tire. Rotate the tire and measure the clearance on both sides. Add to your current backspace/section width to see what fits.