There shouldn't be any reason to change the heater core flow. Once the coolant exits the water pump, flow through the engine is identical to the coolant routing of the clockwise-rotation pump. The fitting in the intake manifold just below the thermostat will still get pressurized hot water, and there should be a return fitting in the pump inlet for the heater return line. Some GM radiators route the heater outlet to the downstream radiator tank. Way back in the dark ages, Ford flathead V-8's had a screw-in fitting in a hole in one of the lower radiator hoses for a heater return, but I haven't seen one of those in at least 40 years.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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