Oh crap..... Internally balanced engine with an external balance flex plate?!

Rapid Robert said
Way back I had a 360 (OE external-both ends) & after new pistons etc I had it internally balanced with the OE external balance dampener up front and a neutral torque converter/neutral flexplate on the back. If I think of it I'll ask him if using a neutral dampener will let the bobweight come in lighter

Well, you did not have it internally balanced if it was an external balance damper up front; the weight changes were necessarily inside but the front half of the engine would still be considered external balance. (The whole term of 'external balance' is a bit misleading anyway; it is just a setup where part of the counterweight is external.)

And the bobweight has nothing to do with the counterweight locations, internal or external; bobweight is all in the pistons/rods/pins/etc. If you want to see, go to the Eagle site in the technical section and look at the bobweight computation example they show. Maybe what you are thinking is if the neutral balanced damper could allow no material added to the crank? The answer is that you would need MORE weight on the crank counterweights.

The way to have less material on the crank counterweights is to lighten up your rods/pistons/pins enough; that change lowers the bobweight against which the counterweights are balanced. We did this with our 340 and a 273 cast crank: part of our piston and rod selection was to get our piston/rod/pin weight to be much lighter than the 273/318 stock weights, and so only removing crank material was needed. As I recall, 80+ grams was taken off of each end of the crank, mostly in the outermost counterweights, but also a bit on the second outermost ones.

What is bothering me about having so-called external balance on one end and neutral balance on the other end is that the counterweight addition/removal on the one end will be different on the other end by a significant amount. This will end up with different counterweight moments on one end versus the other and some degree of end-to-end rocking vibration. So you could well end up with some level of an unusual vibration that you normally would not have.