Exact same issues with the wife's 440 Six Pack Challenger.
Not sure if it's a coolant flow or air flow issue or both.
Engine is fresh mild build with stealth heads.
I have the timing set very mild and the carb(s) jetting/adjustment seems good.

Outside temperature is a big factor.
In cooler temps with low humidity no issues anytime.
In hot, humid 85 plus degree weather is when the engine heats up when sitting.
Driving/moving around in any temp it stays at 180 all the time.
When it's hot out if sitting idling or stuck in heavy traffic temps will creep up eventually getting to 200 plus.
As soon as the car gets moving again temp gets back down to 180 pretty quick.
Engine temps always stay at 180 on the highway.

I've tried a high flow 160 thermostat with new factory custom built 26" 4 core radiator using a factory shroud, aluminum pump housing and HD water pump with 7 blade factory fan with no clutch.
Then went with a new aluminum 26" radiator and high flow Flow Kooler water pump with coolant recovery tank/system.
This made a slight improvement but temps still creeped up when sitting at idle on hot days.

So what is working the best is I added a pusher fan in front of the radiator.
I currently have the fan on a toggle switch and turn it on when the engine temp gets just under 200.
The fan gets the engine temp down pretty quick and the temp stays down as long as the fan stays on.
When I do tuning on the car I turn the pusher fan on and can work on the car sitting at idle indefinitely.
I would like to get an adjustable fan controller installed and keep the toggle switch as a backup.