I can say with some experience that prop balancing isn't the only thing to look at.  If you're using a digital balancer it is not hard to put the second accelerometer on the rear of the engine which can point out vibrations that might not be solely prop based.  And ask to look at the frequency spectrum for vibration other than crankshaft rpm.   The alternator on the continentals turns around 7000 rpm and when it is out of balance all the prop balancing in the world won't help.  Other multiples of crank speed exist as well.  If I was doing a balance I would measure the main rotating components not just the prop and find a good shop for alternator balance if they are repaired.

Paul
Legacy 186