You are in luck as I posted photos when rebuilding the heater boxes in my 64 Valiant Conv and 65 Dart, in parallel. I also had a spare box from a 1964 junkyard car, so when you see 3 air doors in the photos, the odd one is the 1965. All are non-AC boxes and all of the cars had manual cables (instead of pneumatic). They do have several differences in the placement of several of the internal doors and their design. The cable attachments also vary a bit, so look close to decide if you could make a 1965 box work. I also give some of the PN's for the assemblies. All you have to do is search for my post (works better for me from google).
In both cars, I installed a vintage used under-dash knee-knocker AC unit. It was very difficult to get the knee-knockers to just barely fit and not actually knock knees. I know they were a common retrofit as AC became more affordable, but must have been a bit klutzy install and earned their name. If I did it over, I would have just bought a new aftermarket heat/AC box (~$400 ebay), though probably couldn't use the dash control cables without some very clever engineering. Still, it avoids the silliness of have 2 separate blower motors.
I also installed an automatic heater water shut-off valve in the engine bay on both. Probably not needed since the heater box blower won't run when AC is on, but wasn't too hard. I tapped a wire inside each knee-knocker that is hot when AC is on (not just AC clutch) and ran it thru a spare bulkhead terminal to the engine bay where it turns on a vacuum switch which closes a little water valve (newish Ford, $8 ebay, spring return).
