Dart Running Hot?

I have a 70 340 4 speed Swinger that has a stock engine other than the intake and headers. Before I had the engine rebuilt the factory temp gauge ran about 3/4 of the way up. Since the rebuild, the gauge pegs when warmed up. I installed a manual temp gauge under the dash and it reads 210 degrees when warm. Is this too hot and if so, how can I fix it? I changed the sending unit on the stock gauge and installed a new 180 degree thermostat. I am afraid to run it on these hot days because I am afraid of hurting it. The temp stays right at 210 the whole time I drive it. Any help out there?