vivivivivibration

OK, will take a shot at this. nm9stheham is on the right track with diagnosing this. Always fun to figure things out. I call this "Process of Elimination".

OK you state that you have a 1971 340 with a steel crank, that would be internal balanced. 4 speed car so it has a Flywheel and not a torque converter for the automatics so that rules out the torque converter part. That sounds correct so far, the 72 and 73 340 had the external balance cast cranks . . so lets count them out. The internal balance 340 steel crank should have the same front balancer as the early 318s, not needing any special balancing on the balancer because it is an internal balanced crank just like the early 318s. So if your front balancer has any type of lopsided weight on it or a cut out on the back side of it like 160 degrees, then it is the wrong front balancer.

I am going to assume that you have the correct front flat disc style balancer on it. So now next consideration is the Flywheel. Everyone on here knows how 50 year old parts and many different people working on things over 50 years can get things mixed up.

Thinking that you have the wrong later 72/73 340 flywheel on it, 3 shallow holes on the flywheel to purposely imbalance it for external weighting. Need the flywheel without the 3 shallow holes drilled into it for the internal balance crank. Should be able to pull bell housing inspection cover, look up in there while someone rotates the engine over slowly to get a look. Use of some small mirrors and good light will help with the task.

Knowing how things go, could have been an automatic car converted to a 4 speed putting in the wrong flywheel at that time. 3 speed manual to 4 speed manual upgrade with wrong flywheel. You guys get it, things can go wrong. Maybe needed a resurfaced flywheel and had good one around but not knowing it was for an external balance crank and put it together.

Hope this helps . .