The only difference between a 64 box and a 65, 66 box is the resistor.
The later models have a small cut out in the cabin side of the box for the resistor to fit in place.
The 64's had a 2 speed fan which requires no resistor.
You might be able to get the shop to swap the front cover plate as that is the only difference.
65 heater box.
https://www.google.com.au/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http%3A%2F%2Fwww.lamustang.com%2FShowItem%2F102729%25201965%2520-%25201966%2520Mustang%2520Heater%2520Box%2520Assembly%2520-%25203%2520Speed.aspx&ei=_JiMVK5B06TwBcXzgrgL&bvm=bv.81828268,d.dGc&psig=AFQjCNEl0--CpzyksKlWYbCLajBgCWxEhw&ust=1418586733988460
The later models have a small cut out in the cabin side of the box for the resistor to fit in place.
The 64's had a 2 speed fan which requires no resistor.
You might be able to get the shop to swap the front cover plate as that is the only difference.
65 heater box.
https://www.google.com.au/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http%3A%2F%2Fwww.lamustang.com%2FShowItem%2F102729%25201965%2520-%25201966%2520Mustang%2520Heater%2520Box%2520Assembly%2520-%25203%2520Speed.aspx&ei=_JiMVK5B06TwBcXzgrgL&bvm=bv.81828268,d.dGc&psig=AFQjCNEl0--CpzyksKlWYbCLajBgCWxEhw&ust=1418586733988460