After A/C Add On Engine Overheats Really Bad

An engine that operates at too-low a temperature is a bad thing ! If it doesn't get "hot" enough , it won't be able to burn-off unburned gasses and other impurities .

Please note that MoPars equipped with either an axle package or factory-installed air conditioning had :
- 22" or 26" wide radiator ;
- fan shroud ;
- 5-paddle or 7-paddle fan ;
- viscous clutch or torque-drive clutch (one-in-the-same ?)

160 degree thermostats have no place in any post-1965 vehicle ; same goes for less-then-12 lb pressure caps .
Run a 180-185 degree t-stat (get a "fail-safe" 'stat) and a 12 lb - 16 lb pressure cap .

The temperature gauge *should* read some-where in the low-middle to absolute-middle ; this is normal .
One of the reasons --if not thee reason--why your Dart's engine's operating temp is climbing so rapidly at idle / low speeds is because of that direct-drive flex fan .

Best of luck ! Is that your Dart in your Avatar ? If so , it looks quite nice !