Well, it's a "A" code so it could be, does it have the GT light kit? the wiring hole through the radiator support should be pressed and not drilled. Also they have a special steering box with certain numbers I can't remember right now. Also should have the baffle pipes exiting through the rear panel
There is no fool proof way to tell, they can be faked, but only with a lot of work.
No 65 or 66 Mustang came with rack-and-pinion steering, that first came out in the 1974 Mustang
Check in my group files for more