1979 Nissan 280Z Coolant is dissapearing

Engine Cooling problem
1979 Nissan 280Z 6 cyl Two Wheel Drive Automatic 114000 miles

My car loses about a quart of coolant every hundred miles. It has problems cold starting and blew a heater line that has since been fixed. Last week it shot coolant out of the resevoir tank but was depleted in the radiator. The radiator does have a dent where the drain plug is but doesn't appear to ever leak. Although I have considered it being the head gasket, There is no oil in the coolant or visa-versa. I doesn't smoke and drives fairly well. It will over heat a little on the highway but not very much. Any help would be very much appreciated. Thank you.