I have installed a Bosch coolant pump to circulate coolant through my WVO heat exchangers.
My shop manual has a diagram indicating coolant flow to the heater core when the engine is cold.
Does this flow direction reverse when the thermostat opens?
I do not want to try to circulate coolant against the engineered flow. I just have a feeling that when the engine is warm the coolant should be expelled through the top of the cylinder head.

head coolant travels forward so you can put return side of heater to it ,coolant travels same way through by pass wether hot or cold thermostat allows some to go through rad

Please tell us more about the pump,heat exchangers and how you fitted them....if you get the time !