Check the throttle shafts for leaks with the carb cleaner. The shafts become worn allowing air to enter the carb. If the throttle shafts are sucking air you will never be able to correct the lean condition that causes the coughing and spitting in the carbs.
Yes you plugged the big leak, but air will find a way in from some other point. It is still sucking air. Did you check the connections/hose to the brake booster that come off of the intake manifold?
Chasing air leaks is kind of like playing wack-a-mole. Make a list of every connection and joint on the intake system and rule each one out.
Actually there is a procedure for diagnosing engine problems. For example, before checking the intake/carbs etc you should check and set your ignition system so you know if there are problems and have the timing/dwell/plugs etc mostly dialed in. After you correct the carb/intake problems go back and recheck the timing/dwell etc. Otherwise you are trying to diagnose a problem while dealing with too many variables.
When checking for leaks with carb cleaner, did you lower the idle speed to better hear the RPMs pick up as you spray the joints, hoses, throttle shafts etc?