The OP said; "To me, it seems as if the fuel is literally boiling over out of the carb." That doesn't sound like vapor lock to me. It sounds as if the fuel is being boiled away in the carb which would point to a super heated intake.

My thought is that ANY exhaust through the intake given the OP's problem could be too much. Which is why I asked if you thought that there would be no exhaust flow without the flapper. It would seem to me that the only way to know whether the exhaust cross-over plays any role in the OP's issue would be to make sure that it is blocked completely. And the OP won't even need to pull the intake to find out. A temp gun will tell him what he needs to know. Either the cross-over area is significantly hotter or it's not.

I am focused on this issue because I have seen it so many times. Quicker ignition advance and blocking the heat cross-over are the first two things I always recommend to guys that come to me with any problems like the OP is experiencing.