Headgasket would be my first guess, seeing white smoke. Know anyone with one of those testers that goes in place of the rad cap and then you use a manual pump to pressurize the system. That usually tells the tale…..
Also, check the coolant pipes under the carbs. They have large “0” rings at the ends where they connect. If they “drool” under pressure, the coolant can dry up on a hot motor and create a mystery.
And, look at the water pump design.the shaft has numerous seals that separates the oil in the crankcase from the water in the pump. Water in the crankcase does not always mean a headgasket! Could easily be those ceramic seals.
Finally…… check the coolant overflow tank for leaks too. They do crack and leak at +50 years old….. Good Luck!
