vivivivivibration

Early 340 are all internally balanced, and used the same damper and flywheel as the 318. The later 340's had external balance weights on the damper and 3 shallow holes on the flywheel to purposely imbalance it for external weighting, to go with the later 340 cast crank. The later 340 external weighting is different than the 360.

So, OP do you mean that the crank that went in was a '71? Or just came from a '71 block? Any crank could have been put later in any block, so you can't go by the block