Maybe in Canada winters boiling water would work oaky, anywhere else above freezing I would and do use cold tap water and let the motor get up to normal operating temps before letting that water dribble in the top of the carb at or above 1500 to 2500 RPM so it will get mix in with the air and fuel and not drown the spark plugs out and cool down the combustion chambers to much steam at once also
I was around cars a little in the late 1950s when flathead where still being used and hot rodded and the pump gas wasn't that much of a worry in those motors unless they had been hopped up a bunch
I think the lead additives where being introduced and use in the late 1950 as the horsepower wars where being started, that cause the increase in octane and the increase price of gasoline. I remember pump "high test" being .19 cents a gallon and regular gas costing 16 or 17 cents a gallon depending on which brand gas you bought, that was in Blythe, SO CA in 1959 and 1960
It sounds like you need to lean down the carb idle mixture and part throttle cruise mixture or maybe use a hooter spark plug range or maybe both