How do I tell if my 340 is internally balanced?

TMM, I'd love to sit down and discuss it all with you. The above has confused a couple of items.

The balance is between the bobweight PLUS the weight on the crank throws/journals on one side, versus the weight on the counterweight(s) on the other side. The bobweight comes from the pistons AND from the rods. So you end up with pistons, rods, and throw/journal weight on one side versus the counterweight(s).

So, you can indeed pull it off of the throws/jounals to compensate (to some degree) for heavier pistons; that is exactly the same as taking weight off the rods' big ends, and have 2x the effect of taking weight off of the pistons/pins/rings/small ends.

And that is EXACTLY why the 340 cranks has holes drilled inside the front and rear journals.... the heavier 340 pistons meant too much weight overall to easily balance with the existing 273/318 forged crank, so they took metal out of the inside of the crank throw/journal on the 340 versions of that crank to compensate for the higher bobweight from the heavy 340 pistons. Removing weight on one side has EXACTLY the same effect as adding weight on the opposite side. So removing weight on the throws/journals is the same as adding weight (like heavy metal) on the counterweight side. That helped them avoid adding any more weight to the crank on the counterweight side; they just had to to drill it out of the throw/journal.

And BTW, lighter pistons does not require taking weight off of the pad on the big end of the rod... they can both go lighter or heavier or in opposite directions. No fixed relationship there...