I am new here after aquiring a 65 C-30 dually with a dump bed, (Which I do use for hauling) and I had the same issue with the heater.
I did the new water pump, 195deg thermostat in a late 70's 250), and flushed and backflushed the heater core. I was also thinking about running a hose of some sort from the suction of the blower motor, rigging it up to use heat coming off the exaust manifold (like the old VW's) and use pre-heated air to blow over the heater core. It takes a lot of heat to make really cold air hot. I hope this helps. As I am new here, any help with my new ride will be greatly appreciated.
