My 1973 F100 Custom, w/ inline six and c4 tranny w/ no other options came with the heater core plugged. I have replaced the heater core, old one looked real bad. I have a heater hose fitting on the thermostat housing and one the water pump, which goes to which heater core tube, looking at it from the front of the truck?

 

it does not matter which one. but factory the one from the water pump went to the tube on the left side the one form the thermostat went to the right side.

 

If they are both the same size you can swap them, I used to swap them if the heater core started to give weak heat, it would work as a backflush, does a pretty good job too, takes a week or so but you do notice the difference if you have weak heat.