spark plugs

I high efficiency 3-row core will cool circles around a 4-row. Most 4-row cores are too dense to allow proper air flow in town.

If you are running a flex fan, thats part of your problem. If you are running an electric AND a mechanical fan thats part of your problem (run one or the other, NEVER BOTH).

Thermostat rating will not make any difference, since the engine is getting warm enough to open the thermostat. Once the thermostat is open, it is out of the picture. Now if the engine gets hot and the top tank of the radiator stays cool, you need a new thermostat.

Try this: Turn on the heater full blast when the engine is around 225 degrees or however hot it gets. If the temp goes down, your radiator is the problem, it simply can't shed the heat. Do this at highway speed, that way you have plenty of airflow, (this eliminates the fan as a cause, since the fan is pretty much along for the ride past 30 MPH.)

Mopars were never know for overheating, and in fact they had pretty robust cooling systems. If you are running that hot, something is wrong.

And if it runs better with the hotter plugs, you are either too rich, burning oil, or have a puny ignition system (or you just needed new plugs and thats why it ran better). Hotter plugs tend to "glow" under high loads and will cause pre-ignition. Higher octane will not compensate for this.