Have you used carb cleaner at the intake mounting of the adapter, throttle linkage of the carb, all the vacuum connections, including that hole? What have you done with the charcoal canister? Are all the different vacuum points on the intake manifold and exhaust manifold used or sufficiently blocked? Spray carb cleaner.
Lastly, keep in mind that there could be a problem with the carb!!!!!! I went through two carbs from the ebay guy before I gave up on what was a never ending battle with vacuum problems. First carb after one month of back and forth emails he sent a replacement. After problems with the second one he stopped responding to my emails. Got one from an econoline van with a 351 from car quest and its is perfect!!!!!!!
Who knew where this would go?