Ok, good guage, where is the sending unit located? Stock position or elsewhere? It makes a difference. If it's a mechanical guage you can't put the sending unit in the stock location, especially if you have AC. But as long as it's in the front water crossover it should be reasonably accurate. I've got a 65 Cuda, close enough to your Dart to be useful.

There are several things that can cause a high reading without it actually being "real". Years ago I had a Diplomat with a 360 that had high water temps without any other symptoms. Turned out it was an exhaust flapper issue, the spring that opened it up broke so it stayed shut. This would cause the exhaust to always go thru the crossover in the intake, heating it up and causing the temp guage to read high.

But to be honest I would not be concerned about 210 temps.