I am NO EXPERT, at all, but after reading this, if that were
my truck, while the radiator is out, I would disassemble
the balance of the cooling system and CLEAN / FLUSH IT ALL.
Even if you had it open before, open it again and FLUSH IT ALL,
forwards and backwards AND THEN DO IT AGAIN.
The very first thing I did to my M925A1 / NHC250 was to service the cooling system.
Here is what I learned about my NHC250.
You HAVE to flush the heater core FORWARDS AND BACKWARDS several times to get the yuck out
with a high volumn hose and proper adaptors, not just a rag wrapped around things.
Replace cap.
You HAVE to make sure the long small diameter hoses across the top of the engine are CLEAR AND
IN GOOD SHAPE. I simply replaced, easy to do.
I think you said you replaced the thermostat. Still pull it again and FLUSH ALL
forwards and backwards and left and right and do it again!
I do not remember reading you saying you flushed the block as I would.
You HAVE to pull the inspection plates on the side of the engine and flush till
you are blue in the face! Up, down left right and do it ALL OVER AGAIN!
You HAVE to add drain down petcocks and flush forwards and backwards at
both sides at the bottom of the radiator!
You have to disassemble EVERY LAST THING COOLANT RELATED then clean
and flush EVERYTHING all at one time, not one section at a time and put
it back together and hope for the best.
You HAVE to make sure fan components work as advertised.
When you reassemble I think I filled w/ water first and tested for leaks
then I drained down for the last time using my 2 new radiator butterfly
petcocks an filled w/ antifreeze.
All is well...when the fan engages it still scares the begesus out of me
but it tells me its working!
You should listen HARD to Will Wagner FIRST, IMO.