as long as it stays there, it should be okay. I believe most classic mustangs are supposed to run slightly cooler than that but so long as it isn't overheating, should be fine (my cadillac runs at about 210 degrees at all times, and a lot of newer cars run even hotter than that for emissions purposes).
210 is no sweat. It ran hotter than that in it's original application. The roller 351 I had in a 95 E150 once got hot enough to "vapor lock" out in Nevada and did no damage to the engine. It had no numbered temp gauge, but the engine fan wasn't working while pulling up out of Boulder Canyon with a trailer at 10 mph.