Seems like there might be some confusion here over internal vs. external balance.
His 383 obviously uses the 400 crank (not a dedicated internal balanced 383 crank). It is EXTERNALLY balanced.
That said, I don't see why it would not be balanced by adding the same amount of weight that had been added to the flex plate since a flywheel with no balance weights is NEUTRAL <img src="confused.gif" border="0">