I just stuck a 350 in my '60 F100 and up until now, have had the heater fitting in the manifold looped back into the fitting on the water pump. It's time to hook it all back up and I'm wondering if the heater core inlet and outlet are specific (directional) or if it doesn't matter which hose goes onto which fitting.
I wouldn't think it matters but I don't want to find out the hard way.
Nope, the separate water pump tube is a bypass of the thermostat that allows coolant to flow out of the manifold through the heater and get recirculated into the engine when the thermostat is still closed keeping coolant stationary in the engine - warming up until it reaches operating temperature!
So you get hot water right off the engine to the heater and then it gets sent right back into the engine to warm up some more.
Thus you'll get hot coolant to the heater core long before the radiator ever sees any.