You do not mention what year Chevy II you have but the early cars are known to have heating problems when everything is not perfect.
If your temp sender is not the issue, make sure the cooling system is clean and not plugged anywhere, the timing is not too far advanced and the carb is not too lean. Any of these can contribute to over heating. Worse case find a radiator from 65-67 as they are wider and have more water capacity.
