I'll tell you what I have on my 67, maybe you can reference what I use. On the front I am running a 225/50-15 with a 4 1/2 inch back spacing. I am using 620 lb. coil springs, so the ride height is lowered by about 1 inch. I have no tire rub and the top of the fender well is 1 inch above the tire. A 1 inch taller tire would probably fit with the fender lip rolled using the 4 1/2 in back space. and, I forgot to mention, 8 in dia. wheel.
On the back, again I am using a 15 in X 8 in wheel with 4 1/2 in back space. 245/60-15 tires. Had to roll the fender lip at the top and the tire is sitting even with the top of the wheel well. For the rear ride height I have 1 in lowering blocks and standard eye 5 leaf leaf springs. This gives the car, using the above, a slight rake, lower in front.
There are a couple of places in the inner rear wheel well to be aware of. The inner well at the about the 10 o'clock position, looking straight on (driver's side,US), 2 o'clock (passanger side,US) and the rear axle bumper/bracket. I have seen in magazines where some guys are using the 275/60 series supposedly without any clearance mods, if they did the backspacing was well over 4 1/2 inches on an 8 in wheel.
I think Mustangs Plus's web site may have a chart broken down by model years for max. tire size,
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I hope some of this ranting was of benefit to you. ---Chuck
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