Engine gets hot at idle

[Dead Parrot]

When you mentioned the PO swapping from a diesel, I wonder if the PO had budget issues. That radiator could have been the "cheapest thing available" so the PO could get a truck that moved under its own power. Your initial thought on the water pump impeller is worth checking. If the current engine was a salvage yard pull, no telling what shape the pump is in.

My truck came with a factory fan clutch. A lot of trucks lost them in the 70's and 80's due to flex blade fans being sold as a way to gain HP and improve gas mileage.