'59, with Rochester B carbs, it's possible the airhorns are warped enough to leak the power system vacuum signal internally, which can cause the power valve to always be open, causing the carb(s) to run very rich. The power system adds fuel to that metered by the main jet, so that might explain why dropping the main jet sizes wasn't effective. You may have already checked, but airhorn flatness would be an item to investigate on both carbs, if not already done. Unfortunately, tightening the 4 hold down screws can just worsen the warp.
Are the two carbs from a 235? Old-school wisdom was to drop down to the smaller 216-spec carbs if running 2 on a 235. That also gives some good options for Carter W-1 or Stromberg BXOV-2 Chevy-spec carbs that were run on 216s. Those carbs have the smaller SAE2 flange, with 2-11/16 bolt spacing, so if your manifold is for the larger (SAE3) flange carbs used on the 235 with 2-15/16" bolt spacing, you would need to make some further changes.
Just some food for thought... hope this helps.
