Re: inline 6 overheating
Did it run hotter before the w/pump was changed?
Not enough coolant as in needing to purge more air from the system,
Fan belt loose,
Idle RPM too low,
Radiator clogged, either/both, externally and internally - flush with garden hose,
Initial timing too retarded,
Leaking head gasket - do a coolant lithmus paper check for exhaust fumes in coolant,
Had you not added the phrase "when it sits" and just said "when shut off," I would say this condition be normal due to heat soak - no air moving and engine soaks up heat.
But you said when sitting and I assume the engine is idling, so here are my suggestions.
Others might add more suggestions.
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