I fought this problem for about 2-3 months when i did my build last year. Turned out i screwed up one float adjustment and one side was going dry. I set them the highest i could and problem was solved. Although i dont hold her WOT for miles.
What you can do which you might of already have done. Drill a hole strait through where the needle for the floats go ever 90 degrees therefor adding only 2 more holes. It would form an "+" . I did that as well as made the holes a little bigger to help fuel flow. I also put a duel pingle but as you said, that isnt your problem.
If you need pics of what i did, let me know. Ill dig through my old threads and find a picture a member on here posted. I'm sure your cub sucks more fuel than my 4mil though. I've got 162 or 165 mains i believe but each bike is different.
I personally was a little scared to cut a bowl out even though it's a common thing to do. I dont see mine ever going dry again but if it does, a float will be taken from each.