It all comes down to overall wheel diameter and vehicle height.
The aim is to fill the wheel arches to create that muscle car look.
If the car is lowered say 2 inches then the maximum recommended diameter wheel would be in the order of 16 inches with a medium low profile tyre. Rim width of up to 7 inches will work as long as the backspace does not exceed 4 inches (front suspension may interfer otherwise).
Personally I think a 15 inch x 7 inch is ideal for the '65 & '66 stang and these are available in the traditional styled steel just like the original 14 inch x 5 inch wheel.
If you want a more radical look, then 17 inch x 8 inch can fit with low profile tyres but you may have to trim the rear wheel flange near the rubber bump stop and possibly only lower the car 1.5 inches.
Check out this month's Mustang Monthly magazine which has a ghreat article on this very subject.
PT