Dehumidified Defrost needed?
While dehumidified defrost is nice, it is not necessary. Think heat only cars, no AC to dehumidify! Defroster works just fine.
Since you have a sort of blank plate There is a AC unit being sold that is a bolt in replacement for the stock heater only heater box
1970 Dodge Dart Swinger Air Conditioning Kit | 70 Dodge Dart Swinger AC