Trying to convert an A/C heater box to heat-only

I've been running the original AC/heater box in one of my Darts since 1978 with the rest of the AC removed. Heater and defroster work fine with the original controls - I covered the AC duct to the old under dash AC vents, pulled the AC evaporator core so I could put caplugs in the firewall. No muss, no fuss.