The 1964 1/2 1965 boxes are different from the 1966 boxes. The earlier ones had a 3 wire heater motor, and no resistor on the front of the box. The 66 had the 2 wire motor and the resistor on the front of the box. The 67 68 also had the 2 wire and the resistor, but the heater door opens and closes up and down instead of left and right.
The back of the heater box (goes against the firewall) is the same for all 4 years, except the the 67 68 with AC, which is totally different.
Ford apparently had different suppliers for the heater boxes, as the front and back fiberglass clamshells have different recesses to attach the clips to hold the box together. The recesses have about 3/4" differenences at times.
This spring, at the Pate swap meet, I bought a total of 15 heater boxes from various sellers. Between broken fiberglass, rust, other damage, about 4 went in the trash. The rest I cleaned up, and my son sold most of them for fund raising for his Eagle Scout Project.
One guy tossed in FOR FREE 3 67 68 indash air units, in various conditions. After 3 tries on ebay, no interest, they were taken apart, the cores removed and sold for scrap, and the shells went in the trash. Kept the blower motors, as they all worked.
There is also some heater hoxes that are the same, or similar, on Comets, Falcon, Fairlanes, etc.
ed ay +3112
1968 GT500 Arrest Me Red (The Big Boy Toy)
1966 K Code Vert (The K 'Vert)