My guess would be too much carb and not enough runners in the head. Vacuum secondary carbs would help, at least it would lend more adjustability to the setup.

It would seem the engine could handle a bit more total advance, how much do you have in intial advance, and at what RPM is all the advance in? IMO the engine should hand 32 to 34 degrees total advance. Hopefully the chebbie guys will be along with all the exact numbers.....