Two types of heaters. External and recirculating. The cores in both eventually rot and start leaking and typically caused the floor board below to rust. The solution was to "horse shoe" the heater hose back to the water pump/thermostat and bypass the heater. If you really had to have heat, scab on something that works. On a farm, these trucks were treated rough.
"Adding CFM to a truck will only help at engine speeds you don't want to use." "I found there was nothing to gain beyond 400 CFM."
