on the mustang depot website, im sure all of the wheels are the correct backspacing....since the site is for classic mustangs. You could shoot a PM to MDJAY....he is Mustang Depots Rep here in the classic forums.
however, Gellpak is right, ebay is the way to go for wheels like TTIIs. Just make sure you have a 4.5-4.75 BS with a 5x4.5" lug pattern (if your car is a V8)