I'm just really surprised that 'something' is enabling the engine coolant temps to slowly creep north of 180* when idling - assuming everything is 'good' and functioning correctly that just defies logic. Following that same logical train of thought, if the radiator isn't the problem...then something engine-related is, and my first thought there is I wonder if the t-stat isn't opening up far enough. I know it is opening because as the engine warms up the water temp 'parks' firmly at 180*...for a bit...but then slowly begins to creep up...even just idling. If the t-stat isn't opening far enough then of course there isn't enough coolant entering the block fast enough to do the job.
Further downstream...if it isn't the t-stat, then I think 'water pump' - it now has a fancy 8-rim serpentine pulley, which has a nearly identical diameter to the OE v-rib pulley it replaced, and the crank pulley is also nearly identical in diameter to the OE unit, so the water pump speed should be pretty good.
If it isn't either the t-stat or pump...well then maybe a blockage somewhere - pretty certain I didn't leave a paper towel in the passageways, and I'm sure I'd have noticed some sort of 'noise' if the water pump was trying to pass something through it other than coolant...but I've done dumber things...
I suppose...if/when I have to drain the coolant, I'll discover (or not) bits and pieces of paper towel pulp in the bucket.
Another thought is the fan clutch - is it "clutching" - warming up the coil and spinning itself up to increase fan speed? Dunno - I have not noticed that semi-truck "whooOOOSSHHHHH" I hear routinely from my '07 Cummins 5.9 when it wants a bit more cooling.
Also...'depth of the fan within the shroud' - I probably have about 1.5" between the front face of the fan clutch and the radiator - no idea of that's good or too far away - I can space the fan/clutch forward deeper into the shroud, which might increase airflow through the shroud.
So, lots of things to ponder, the non-invasive stuff will happen first, and then I 'dig in' and see what's going on...which'll suck because nothing is leaking! 😣 , but gotta do what I gotta do.