Did you convert the switch to a heater only or does it have the A/C controls on it too? See on the 69/70 (what i'm most knowledgeable in), the non-a/c cars had cable only controls to switch from floor to vent and hot to cold (had electric for on/off though). The A/C cars had the floor to vent operated by vacuum, thus the vacuum line coming through the firewall and going to the heater control valve. Non A/C cars did not have any vacuum lines for the heater system so you obviously wouldn't have this valve. It is OK to leave it on, as it stays open when there is no vacuum applied. Not the prettiest thing, so if you want to tidy up your engine bay you might take it off. HTH