Yet another question on vibration (Not the good kind)

OK, a 318 is internally balanced, even with a cast crank. A cast crank 340 is externally balanced, and requires a harmonic balancer and flywheel specifically for that motor (or at least a flywheel that is properly drilled for a 340 cast crank).

Although I haven't tried it, I'm pretty sure a 318 cast crank would need rebalancing if used in a 340, with its larger, heavier pistons and (I think) rods. Maybe yours was balanced when the motor was first assembled with that crank. In any event, if your replacement cast crank was a 340 crank, then definitely you'll need the cast 340 damper and flywheel. Or switch to a forged crank. Or mallory metal to internally balance your cast crank. If you want to keep it an externally balanced motor, note that only 73 and some 72 340's used cast cranks, so the cast crank damper is probably hard to find today. The flywheel, well, I think the Mopar Performance Manual tells you where to drill a neutral balance flywheel to bring it to cast crank 340 specs. That's my memory, anyway, and my manual is out in the garage, so I don't have it before me.