this is my 65 with 18x9.5 on rear and 17x8 on front. Lowered 1"...still needs more lowering. Trying to figure out best way to do this. Fenders are not rolled nor is there any rubbing, except for the parking brake cable.
Tire brand makes a big difference in the actual width as well as contact patch
Check out the differences in tire width and contact patch for the same size tires only different brands. Unless you've looked before, I think you will be suprised. My 235/45s are as wide with a bigger contact patch as some other 245/45s.
If the rear doesn't compress enough to get the wheel lips into the tires he can go as wide as he wants. That was the cool thing to do in the 70s. Hell we were running N50x15s on V8 Vegas. Because the rear lips are double walled on the Mustang, it's a PITA to roll them, and when you do you only pick up about 3/8" clearance rolled tight (on first gen Mustangs, I've never rolled lips on later model Mustangs).
After rolling the lips as tight as I could, I measuring the clearance between the inner fenderwell and the wheel lip. It measured just at 10" + or - 1/16" on each side of my '66. Perfectly centered with a Watt's link I wouldn't be comfortable with anything wider than 9.5". Body roll and spring bushing/shackle twist is still going to move the body at least an 1/8" under duress. Unless you are building a go straight only car.
That sure is a nice looking car though. Makes me hungry for Ice cream. Mint chocolate chip! Mmmmmm!