Problem fixed: Engine would run hot at idle.

I had already rebuilt the lower unit with new bearings, seals, gaskets, o-rings, and replaced the water pump, housing, and plate. The water pump delivered great of low and pressure, but the unit still ran warm if idles for a long period. I removed the thermostats and diaphragms and checked for a restriction or blockage and no issue was found. Decided to replace the thermostats and diaphragms as they were very old and the engine no longer overheats when idling for long periods.