you are going to have to take the carburetor apart. just speculating and discussing it on the forum isn't really the same thing. Even though carbking says it doesn't sound like the carburetor, checking the float level, (both high, and drop)
and inspecting the needle valve and seat is very easy and will eliminate those issues. It is possible the float is sunk/partially sunk. Whatever the case there is no way it should be leaking out the top if all of the above conditions are met. Which need to be met for it to run properly. Now I'm not saying it isn't something else, but I do know if you have so much fuel in the carb it is running out, the engine won't run. Can't really see how this would do it, but I have had a piece of rust the size of the period at the end of this sentence jam up my fuel pump at least twice. That is one problem with 50-60 year old vehicles, the fuel system is usually all goobered up. I fought rust in the tank for about a year, before I finally broke down and replace the entire system, (everything except the filler tube and the little hoses that connect the filler tube to the tank.)