Don, any tube on any 3800 that exits the lower intake manifold's coolant passage is going to be the outlet (to the heater core). Any tube or port on the water pump is going to be the return (from the heater core).
You do NOT want to plug either of these ports/tubes and just leave them capped off on a running engine. If you are not going to run a heater core, then you need to connect both tubes/ports together with a length of heater hose. The reason why is because if you block these off, the water pump could build extreme pressure if the engine is operated at high RPM while the thermostat is closed - and this could blow out gaskets and seals. Connecting both ports/tubes together with a hose will allow excess water pump head pressure to vent off and recirculate back into the water pump while the thermostat is closed.
OVERKILL IS UNDERRATED
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