[Old Truck Man]

My opinion from Experience is. The heater on a 63 thru 66 truck was marginal when new. The cold air enters the cowl vent at the windshield and is drawn through the heater core before entering the cab. You cant warm up that much cold air at speed when its below freezing. My dad bought a new 64 truck and the cold weather (Indiana) we froze. My temporary remedy is to cover the cowl vents with duct tape. Then open the fresh air vent inside the cab so it will recirculate warm air. The permanent fix is to use a 60 thru 62 heater. and to boot you can remove all that heater box from under the hood. My wifes dad bought a new 66 3/4 ton. He lived in Rockford Ill at the time. The poor preforming heater saved that truck (we still have it). Because he would not drive it in the winter thus it didn't rust out from the road salt.