A lot of things can be done before you add a pusher fan. For a street car you should overdrive the water pump. It eats up more power, but losing a few hp is better than cooking the motor.

You'll also want the best water pump you can buy as well as high quality hoses and belts. Make sure the radiator is the biggest one available for that make and model. Use a factory shroud and the biggest clutch fan that you can fit in there. Late model C body cars had huge fans so if one of those will fit use it.

The temp of the thermostat doesn't really matter on keeping things cool. What you need to do is run as much water thru the engine and radiator as possible. So speed up the water pump, make sure there isn't any resistance in the hoses or radiator and then hope for the best.

Pure water works the best but if there is a possibility of freezing then run a little bit of anti-freeze in the system. The closer you are to pure water the better. Also, run a high pressure radiator cap but make sure your hoses and radiator are in good shape before you do that.

And of course, make sure the engine is properly tuned. Advancing the ignition will probably cool things down. Also, make sure the carb is jetted properly.