It all depends on what you consider over heating?
Do you mean "boil over point"?
The GT500 has a 12 psi pressure cap and system. With a 50/50 mixture of anti-freeze, it won't boil over until around 240F. Anything under that is not over heating if you are talking about boilover.
Do BB Mustangs run hot? You bet ya'! Try one with an automatic transmission with A/C in Phoenix?
Probably the biggest problem is going to be a restart when the engine is at "full operating temps".
Secondarily, with a '67 GT500 in particular, the carbs will boil over immediately after shut down as built and delivered new.
The best solutions to that is to isolate the carbs and to put in a high torque starter. Personally I wouldn't run anything but a 160 thermostat and a full three core radiator.