Why do you want to know the original configuration for the car? If it is to restore it to exact original...do what the big boys do and buy a data plate that makes it whatever you want it to be! Since, as you say, there is no way to really verify such, make it a hipo stickshift....there is at least one company selling trim plates made to order.

Unless you are doing a restoration, you are probably just curious. Perhaps one of the mustang clubs could help with some of the small clues...