Found a heater box from an early mustang
Bought a new seal kit, heat defrost plenum from Mustangs unlimited.
The mustang must have a shorter distance from the cowl to the heater as
I had to use a new foam seal and stack an old used one on top of it.
Other than that the heaters are identical.
Earlier Falcons used a bigger heater core (60-63 was 2-1/2" thick and somewhere after that the cores were 2" thick.
Also----starting in April 65, Falcons and Mustangs got 3 speed heaters vs the earlier 2 spd (applies to the 64-65 cars).
The cars with the 3 speed heater will have a hole cut in the heater box for the blower resistor that the early 65 and 64 cars do not have.
If you replace the heater box (and they are available new from Falcon Enterprises, Mustangs unlimited, and Melvin's Classic Ford Parts , they will all use the same heater core---cannot recall which one.
I think I found that Melvin's carries the large dryer vent hose type seal, but I have not yet ordered one----I will later----now AI have to repair my defrost hoses and vents 'cause they broke while wrestling around with the heater.
Hoses and vents are also available new-----about $40.00
But--my heater is working now and I can actually drive it in the mornings if it not too humid out----defrost only halfway works.