The bowl extension gives more volume to the back bowl where the fuel gets away from the jets. Most alcohol carbs have them, ours do and I wouldn't run without one. Sounds like your car might be a good candidate for one.
Float level should be fine I just think maybe you have reached a point to where you are using up the fuel in the back bowl enough to get away from the jets. It would still be good for you to know the pressure when you are having the problem though. We have data recorders that we use for that.
Curtis