Do you have the correct coolant mix? And correct functioning radiator cap.
It is a big block, and will generate a lot more heat to transfer.
My 1 ton dually new had 200 or 210 thermostat. After 3000 miles I swapped it out for 185.
Going over I-80, Donner Summit, to Reno, in the bed luggage,tack, six bale's alfalfa. Pulling a two horse trailer with change storage area, never got over 200, yeah August vacation.
It sounds like you loaded a parts Cannon and blasted everything at it.
But you haven't actually found the cause.
It runs at higher temp. Does it boil over? Does it pressurize the cooling system beyond normal? Do you experience any pre-ignition.
During a loaded, working condition did the temp go unusually high, or slight equal to increased load ?
What does the service manual reference with this model and build?
What are other similar built trucks experiencing?
Have you checked for Service Bulletins on your model build or any big block trucks? That's what I would do. Good luck