I give up

My 5.9 magnum runs around 200 210 always and sometimes hits 220 before the stat open fully ..then it drops down to the 200-210 range and stays there.

The temp gun is a good idea just to have around for everything.

Flush the rad
pour in the bottle and let it run for a bit-rev'n the motor some then pull the upper hose from the radiator and let it point out the front of the core support with some plastic under it, then stick a hose into the inlet of the rad and run it for a few minutes or so.
I used be able to remove a rad, stick a hose in it, holding hands around neck to seal and pressurize the junk out the the lower 'with cap on of corse'