The supply to the heater core can come off any intake manifold port that is below the thermostat. Here is a picture of the heater supply coming out a port just to the right of the thermostat fill.
The last one it did, I made a tube that went between the passenger side coolant tube and the inlet to the water pump. This tube had a nipple welded to it so the heater core return could merge with the coolant coming from the radiator. This is how the stock 88 Fieros route their heater cores. In this pic, the braided stainless hose is the supply and the rubber hose is the return.
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LS4/F40, HSR/SBC/F23, Pro-Flo/383/Getrag, 4.3CPI/4T60, Ramjet SBC/Getrag, 4.9/Isuzu, Carb SBC/Isuzu, 4.5/Isuzu
