I suspect two causes. You need to know how coolant flows through your engine.
It starts at the water pump, flows through and around the cylinders, continues up the back of the block into the head, then coolant flows forward through the head back to the front of your engine and across to the thermostat. Both sides follow this route independently until they meet at the thermostat.
If your heat riser valve is stuck shut tremendous BTUs of superfluous heat must be conducted away by the coolant system. We normally see steam created where coolant flow is insufficient. Steam WILL come out the overflow.
I suspect you have a problem with the exhaust crossover which goes under the carburetor, not the cooling or air/fuel ports. This crossover gets hot. Check your heat riser valve. - Dave