Do you have a model 3310-1? That is the 780 CFM one with downleg boosters. I have a similar 400SBC. It did not like the 3310-1. The bottom end and mids sucked. The 3310-2 and up seemed were a lot better. They have straight boosters and are 750 CFMS.
I have used a lot of different carbs on my engine. So far the best running carb is the current one. It is a model 84013-1. It is the "New" Holley. It has annular boosters. Supposedly it is 750 CFMs, but judging by the venturi size I think it is closer to 800. It has metered the best all around.
I haven't tried the Demon carbs yet.