Just an FYI, TD, if it has Keith Black pistons in it, then the piston + pin weight is around 140-150 grams less per slug than the stock pistons and pins. That will reduce the bobweight by somewhere in the 6-8% range (just off the top of my head), and that will make it imbalanced... not horribly, but still definitely imbalanced. The good news is that balancing the crank to this lower bobweight will involve removing material from the crank and not require adding Mallory metal.
As for having imbalanced parts at the front and back (damper + flexplate/TC), that is absolutely normal. You should indeed have imbalanced parts at both ends.
And FWIW, 'externally balanced' is a bit of a misnomer..... it really should be called 'partially external balanced'. The cast crank material is lighter, and consequently, the front and rear counterweights can't be made big enough to properly balance against the stock pistons, pins, rods, etc., and still fit inside the crankcase and journal size limitations. So some portions of the front and rear end counterweights (which are the biggest ones) are simply moved to the outside the crankcase.
It sounds like you have a path forward to solving this; let us know how it goes this winter!