Which sort of takes us back to the beginning: Have an overall plan and consider implementing any desired chassis or brake mods before you invest in those perfect-looking new alloys. Thoroughly measuring your Mustang or Ford for a new set of shoes can be a tedious process, but so can trying to sell or send back a new set of wheels and tires that don't fit.
How Much Is Too Much?
The earliest Mustangs wore some pretty scrawny factory rubber-heck, that's all that was out there at the time-on rims no taller than 14 inches (at least until the Shelbys came along.) And their wheelhousing areas were engineered and sized accordingly. These early cars are the most dimensionally challenged when it comes to accepting larger rolling stock, but can still be substantially upgraded. By 1969 and the advent of factory F60x15 rubber, a bit more space was provided. Though individual measurement is still the best way to proceed with your own rolling stock selection, we've stumbled across a website that has catalogued wheel and tire fitment on various years of early Mustangs with various brake and suspension combos. Have a look-but don't throw out that reliable old tape measure . . .http://dodgestang.com/mustang.htm
Also, for a well illustrated look at wheel fitment tips and terminology, you may want to check out: www.rsracing.com/tech-wheel.html
Calibrating For Calipers
Clearly, the decision to upgrade/upsize brake rotors will have implications on minimum diameter when it comes to choosing rims. But caliper design will also play a major role in determining which rims will fit. The following illustrations detail the specs of two different Stainless Steel Brake Corporation (SSBC) kit offerings for early Mustangs. Both have 13-inch rotor diameter and require wheels of at least 17-inches, but one (the three-piston Tri-Power) is of floating-caliper design while the other (the four-piston Extreme) uses fixed calipers. The fixed-piston Extreme caliper (on the left) protrudes 0.92 inches outboard of the hub's wheel mounting surface (this is called caliper overhang), while the floating Tri-Power caliper (above) actually tucks 0.12 inches behind the hub face, therefore giving a lot more leeway in spoke-to-caliper clearance. It is absolutely essential to consider-and specify-caliper overhang when ordering rims. By the way, SSBC also has a 12.5-inch rotor braking system that will work with a variety of 16-inch rims.