FWIW, I have a stock heater in my '53 3600. I installed a 12v motor, sourced I believe from Classic Parts (formerly Chevy Duty). I am not terribly satisfied with the airflow out of the heater. It does warm the cab, but it is very weak blowing air to the windshield for defrost. I would love to have a motor that blows twice as hard. My truck is a year-round work truck, and it can get pretty cold here in west-central Michigan.
