Harmonic Balancer Question....

If you internally balance it, you can use a neutral balancer as well as a neutral converter.
Lighter rods would help too. I can't think of any off hand, but there are very few options for aftermarket external balanced 340's, which use a cast crank.

Any time you start swapping rotating parts around, you need to rebalance. It would just be best if you could internally balance without Mallory Metal by using lighter pistons and rods. It's possible with a 360, so a 340 would be easier because it has less external counterweight at each end.