On my '75 SBC, the hose connected to the fitting on the passenger side of the intake manifold supplies water to the heater, and the return line from the heater runs back to the fitting on the radiator. With the engine running and the radiator cap off you definitely see the gush of water coming in from that heater return hose.
I have a Stewart high flow water pump and it has a threaded fitting on top of the housing (for a heater hose?). However, I didn't need to use it, so I just screwed in a pipe plug.
Bruce