Stan, I would look at on an individual basis in this case- set up a video camera & film the carbs while racing ( or observe them on a dyno pull) to see if the motor goes thru a vibration period that affects the carbs. You may be able eliminate them completely, but with that runner length I imagine the carbs are some distance out from the cylinder head face, or have you incorporated some 'length' into the plenum ( sorry-perhaps I shouldnt ask that
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Russ suggestion of removing the isolaters and machining the manifold face for o-rings has some merit for shifting the carbs inboard a little.