I had overheating issues when I first put my sb Dart together. I tried running electric fans and they didn't work for me. In the end a Viscous fan kit from MP and a good shroud fixed it.

As others have mentioned check the inlet and outlet temp on the radiator along with the temp at the thermostat housing. I like Stant high flow thermostats and also drill four 1/8" holes on the flange.