I own and have owned dozens of the 60 thru 66 trucks. And the heater sucks. especially the 64 thru 66 trucks. The air is drawn from the cowl thru the heater core and even when new they didn't work very good. Having a 160 or 180 thermostat didn't help. My dad bought a new 64 and on a 20 degree day it wouldn't defrost the windshield. What I do is put duct tape over the cowl vents. and open the passenger side vent. That way the already heated air is recirculated . and use a 195 degree thermostat. I have a van that has a second heater core and fan under a rear seat. I think I will install it in my wifes 66 C20.