[DanEC]

Some additional response - if you have stock, reproduction dual carb linkage, it is progressive and the rear carb primaries don't start to open up until about 30% throttle and then they are levered to open faster than the front carb so that they both reach full throttle at the same time. I don't know if it can be modified - or if a non-staged linkage system is available - to run off of both carb primaries from idle up. There is some adjustment in the linkage that you might play around with. After any adjustment open the throttle fully and make sure the primary blades on both carbs are able to open fully. I suspect if the stock linkage is used to bring the rear carb in too early, it may prevent the front carb primaries from opening completely.

If I'm wrong - and it is your secondaries that are slamming open to fast with a big boot in the back, then a couple Holley secondary spring kits should be able to tame that somewhat by going with a stiffer spring to slow down their opening. Holley vacuum secondary carbs are supposed to work without a noticeable power surge from primaries to secondaries to prevent bogging.