There is a slight chance you have a wrong water pump or pump housing..there are some problems with certain 440 pumps currently routinely sold for 413 replacements , where some kind of metal plate or spacer is inside the pump housing and sometimes not. I do not know all the details, but bottom line is any given housing and pump must be made for each other. Add to this the variation in AC pumps (impellor design ) or not.
As I understand it, from memory, a situation can happen where there is a gap behind the impellor to the casting, due to incorrect mix and match, so flow drops . Even if it fits bolts, it might be the wrong one. . Sorry for lack of specifics, it was long ago.
Perhaps see if you have strong flow in the radiator at slightly off idle say 1500 rpm??? also if water is moving well, then generally radiator is almost same temp top to bottom ; if slow top gets a lot hotter. (IR temp gun) Also attempts to make the radiator look nice with shiny black paint are obviously 100% in the wrong direction. Painted fins cannot throw heat well.
Although I have no personal experience, there is a co selling both pump parts claiming hi flows , “flow cool” or something, hi flow water pump ; If it gets to that.. If marginal pump , would expect hot in traffic, OK on highway at higher RPM?
Not sure, of course, if this is valid for you, but had some problems like this in past years with 480” 440 in 57 Dodge, built from various parts. ; overheats at idle, all new stuff!!
I live in south central Texas (San Antonio) I purchased my current 300G last April 2016. The cooling system with AC is new including radiators, hoses, fan etc are all in new condition.. I've installed a recovery tank but the I'm still having problems with over heating on days where the temperatures reach 90 plus. Any suggestions.