How new is this newly built engine?
When I started off with many new engines I commonly come across that problem on startup and on heavy throttle. I don't think there is anything too wrong with the engine. There may be some tuning issues with those carbs on the input. The cam you said is fairly hefty so expect some of that overlap. I think with the bog and the flame you should tune down the jet size. The thing with a Holley is that they use power valves which shoot fuel very quickly on high throttle. So that may be one of the reasons that they seemed worse. That's all for now, I may think of something later or someone else will. Good luck.