I think the temp sensors on the T-stat housing varied over the years. One electrical sensor prevents the cabin blower from running until a certain coolant temperature is reached, to not blow cold air on passengers. I recall a plastic thermo-vacuum one which was part of the emissions system (now gone). A 3rd one might be to actuate the radiator fan, in addition to the temperature switch at the AC filter/drier. There are also 2 temp sensors on the L side of the head. One is for the dash display and recall the other is for the EGR box. Today, they would have one sensor and share its reading digitally.

When I had a "runs too cool" issue in my 1984 300D, I put the T-stat in a pot of hot water, along with a new T-stat and one for my old Mopars. The suspect T-stat opened a bit too soon and responded sluggishly with temperature, whereas the new T-stat opened snappily at 82 C (180 F). In the car, the coolant now runs at 82 C on the dash gage winter and summer. I used both a thermometer in the pot and shot the T-stats with an IR gun. The later was amazingly close to the thermometer readings, even shooting thru the water at a shiny surface, which amazed me.

My 1985 used to run hot at long stoplights, creeping close to the red line on hot days. I tried everything - water pump (2), used radiator and citric acid flushes, T-stat (2), even removed the T-stat housing to see if a rag was blocking flow. Finally, a new radiator cured it. The old one torn open had a thin film of orangish metal corrosion (?) blocking many channels. But you have a new one now. Trying to purge air by inclining the front and cracking the upper hose while running is a good suggestion. Good luck.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans