This may be obvious but what is the flexplate balancing against?
The Nailhead is externally balanced. What that means is both the flexplate on the rear, and the harmonic balancer on the front are weight-offset and add to the counterweights on the crankshaft.
An engine that is internally balanced would have all the required weight in the crankshaft.
That counterweight or offset balance is needed to offset the weight of the rods/pistons.
Tom once posted the Nailhead flywheel/flexplates have 4.5 ounces of offset weight..... that explains the extra metal you see on the stock flexplate.