I lived with the passenger cars and trucks of the years you are talking about, and never had any trouble with heating or defrosting during northern Wisconsin winters. If the heater was working properly, we (in the shop) never had complaints of poor heating/defrosting, even as temperatures went to -40°. At those low temperatures, the most common trouble was no-start: dead battery or flooded engine. Usually the flooded engine was the cause of the dead battery. But, the service truck was warm- a 1958 Chev half-ton 4X4. The other service truck, 1946 Chev wrecker, wasn't so warm with its smaller heater and drafty cab.
If your OEM heater is working properly, you won't have any trouble at the moderate temperatures you are talking about. A common mistake was hooking up the hot water hoses backwards at the heater core; then you don't get the full benefit of the hot water ahead of the thermostat.
Harvester
