Now I'm really curious to see what others with a Non AC 71 small block A body actually have. I just can't wrap my head around running hot water through the heater box in August!
There are three control cables in my car. One for defrost control and Two that work in conjunction for blend air from the cowl and air shut-off from the blower motor. If full cold the blend and blower door's are shut and no air flows over the heater core.
If moved towards the heat side then air flows through the heater core from both the blower side and the cowl side and is directed either to the floor or both to it and the defrost vents depending on the other defrost cable position. The full extent of the Heat cable will close off blend air from the cowl though the passenger box an force full flow through the heater core from the blower. There is always full coolant flow through the heater core and only felt inside when the doors are moved for such! Unlike A/C equipped cars.