My 2 cents. I have a 65 mustang that currently has Center Line rims and BF Goodrich tires. Front 15x7 205/60/R15 Rear 15x9.5 P265/50/R15
I am installing Willwood brakes and my Center Line rims will not work with the bigger brakes. I purchased American Racing Torque Thrust II rims and Falcon tires. Front 17x8 235/45/R17 Rear 17x9.5 P275/40/R17. Rims have a 4 3/4" backspace. From what I've read if you go with 245 up front they will rub unless you roll the fenders. There is a pretty good article in the mustangs plus catalog page 222 on backspacing and rims size. Hope this helps.
I really don't understand the obsession with cramming as much wheel and tire under there as possible...sure, there are practical considerations...more tire is more grip, and larger wheels clear larger caliper/rotor combinations....but in the end, even for those of us building for handling these are not race cars...there are very few vintage mustangs out there that can make use of 18" wheels with 245/40/18 tires. I mean, I appreciate a larger wheel than stock and am myself using 16" wheels with a 215/55/16 tire....but you wont ever see me run anything larger than a 17"(even a 17 verges on cartoonish looking in a 66)...and I would only ever consider a 17 because of the larger rotor I would be able to run....on a stock car with stock drums though? 15 or 16 at the most.