440 6 pack issues

Most of the above advice is good. The end carbs always contribute. Swapping the end carbs to adjust the air/fuel adj is not a good practice. You should get a Promax throttle plate for the rear carb, pay attention when installing it for a conflict in one area. When checking the end carbs to see if they are adj properly, put one of your fingers over the outside air bleeds. Does the engine rpm increase or decrease, adj accordingly for no gain or decrease. It does sound like you may have a center carb issue. Has it backfired, is your power valve good. You probably have sprayed around the carb bases to check for vac leak. What do the spark plugs look like?