I believe all American Racing Torque Thrust have the same backspacing - 3.75 inches, no matter the size. Other brands may vary but why not stick with the original.
Actually, American Racing tells you that the 67-69 Mustangs should have their TT with a 4.25 backspacing. Only problem is that they don't have them in stock and it takes 6 weeks to order them. They only stock the 3.75's. That's where the confusion came in. American Racing (the original as you call them) say that the 3.75's don't work.