Heater valve missing

Bought truck used. While pulling hard with trailer uphill, pedal to metal as only primaries work on carburetor, I blew heater core. Upon inspection I could not find heater control valve. Hoses go direct from port on engine before thermostat to heater and return to inlet on radiator about three inches from top of radiator.
Truck has never run hotter than 145 and stays at about 120 in the South. Have A/C.
This is because heater returns water all the time to rad. And does not stay in engine long enough to heat up. Thermostat is 180 and opens at 185 in pan of water.
Where does the heater control valve go in truck and how is it activated?
Afraid to put in another core, as I will probably blow it. Truck sat up for ten years before I got it.
Will tackle secondaries in separate question later.
Thanks in advance.