Yep. almost always a coolant flow issue...not an airflow issue

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Verify what pulleys you have.

Call Glen Ray

Maybe Not. I had the exact same issue with my 477 Hemi I had a new large Be Cool radiator, very high-flowing shrouded electric fans and still ran hot at idle only. I changed my pump to a High -flow Flowcooler Pump and Robertshaw High-flow Thermostat because I suspected the problem was low coolant flow at idle. The overheating issues were gone after that. I'd try those two first as they are way less expensive that new radiators and fans.

Lots of different problems can cause issues with cooling, huh

mine as well. most of our old rides came with a 185 or more commonly, a 195 degree thermostat. and as has been said, that is the minimum temperature. 200 to 210 will not harm these engines as long as the cooling system is up to task, and the engines oiling system is as well.
yes, i know it ain't 1967.
but if you are looking at your factory instrument cluster telling you your engine is 180 degrees, or the needle is pointing toward the "L" or possibly slightly more in the "middle" of the range sweep, you better take that with a grain of salt, as those indications were just generalities at best when new.
unless you have taken some time to precisely calibrate them to a "master" gauge/thermocouple, and this CAN be done, you are just playing tiddlywinks.
if it doesn't steam like a freight train, or puke coolant all over the ground when shut off, it ain't hot.
remember, this is just my OLD MAN opinion and my OLD MAN life experiences with [very] close to 60 years playing strictly with, and learning what i know now of, the Mopar brand of vehicles.
YOUR mileage will CERTAINLY vary.........

Define "after a while" also what is the ambient temp and other conditions like bumper to bumper traffic or ?.

Have you verified the readings with a thermocouple or RTD to a digital readout?
I have seen NEW "quality" gauges be off by as much as 40 degrees at the top end

removed advice because it upset people who have supernatural engineering insight

So

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I'll agree
With some saying coolant /air flow, try increasing the RPM to 1.5-2K. You will be generating more heat but also moving more air and coolant. it's another data point EDIT: and is simulating driving down the road as far as water flow is concerned, less heat will be generated as the motor is not under a load, HOWEVER more heat will be trapped under the hood

I "start" getting nervous with a know accurate reading until 220°. The general rule is 230° is the danger point and exceeding that may lead to internal damage and that it will definitely occur by by 245°. There are a lot variables due to clearances, components used etc. How hot does a top fuel motor get by the end of it's 1/4 mile run?
I have always gone straight to a thermocouple which is coupled to a laboratory grade digital temp meter. The thermocouple is placed as close as possible to the t/sat and inserted directly into the coolant. One can get extension leads so the meter can be placed on the seat if desired. Many will use surface probes or IR guns. IMO, they should be used as an indicator ONLY as they are rarely accurate.

I have calibrated factory gauges (what a PITA). And as previously mentioned I have seen brand new "name brand" gauges be whacked out of the box. Both electrical and mechanical. Voltage and grounds can affect the electrical ones Interesting tidbit, the gauges always seem to off on the high end and the warmer the reading the further off they are.
I do know from doing the factory gauges, there are two adjustments for the high and low reading. It may test your patience as the "pivot" point between the two is not in the middle. (Think off an offset center on a balance scale).
So adjusting one end, affects the other but not by the same amount that's the patience testing part. I would imagine if one
does so on a regular basis they may develop a "feel" for it or they may be a piece of equipment that the gauge repair people know of.
The gauges are thermally activated by the current passing through the gauge windings. Should someone want to try this themselves, a small box with a clear face will be needed as any air moving by the area will skew the readings, ( IE: walking by, picking up anything off the bench etc.). This does not occur in the car as the gauge is inside the cluster Allow 5 minutes or 1/2 a beer between adjustments for the gauge to "stabilize"

From my engineering days:

I can tell you 100% water cools better but is prone to corroding and freezing ( DUH)
50/50 has a higher boiling point, doesn't freeze except in EXTREME temps
100% propylene glycol will not cool as well but will lower the freezing point

The differences? Clearances, efficiency, materials used, and technology. That's not to say that a 67 engine built with todays technology would be any different. To the OP, way to many variables to answer with any certainty IMO

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