vivivivivibration

Show us a front pix of your damper....that is where the stock later 340 damper is imbalanced. It may look like the right hand one in the 7th pix in this thread:
340 balancer question | Unlawfl's Race & Engine Tech | Moparts Forums

All dampers and flywheels are almost 100% certain to have a few holes in them; these are drilled in balanced dampers and flywheels to balance them. So WHERE on the flywheel the holes are located is the critical part. That is why I linked the diagram; you have to check at a specific angle relative to the crank bolt pattern in the flywheel to see if the hole/holes are in the right area (on the engine side). So you pretty much have to pull the clutch to see the crank bolt pattern to be 100% sure.

If the damper and flywheel are not imbalanced (i.e., for neutral balance crank), then you're going to have to pull the pan to inspect the crank to see if it is forged (early) or late (cast).

OP, what pistons were used in the rebuild? Are they stock or what?