Setting up A/C car for Heat only

Get a cable operated heater control valve. The AC control panel used a vacuum operated heater valve so you will have to be creative on how to hook up the cable.
Maybe put the vents in a set position then use the vent lever to operate the heater control valve.
Swapping in a heat only control panel is the right way to do it but they are much harder to find than the AC ones for the 73+ cars.