I think it would be advisable to find some good publications and study up on the engine. Tens of thousands of articles and books have been written on the sbc engine since it's introduction with the 1955 model.
You have a vacuum secondary carb which means the back 2 or (secondary) throttle plates open at higher engine demand. There are also mechanical secondary carbs that open the secondarys with just the linkage position, usually start opening around half throttle. Those top butterflies are connected to the secondary metering valves I believe so as the air rushing past opens the plate, it also lets in more fuel.
You have to decide what you want the engine to do before you start looking for better performance.