[Pete]

Normally adding a damper to either the front or rear of the engine you will NOT notice any difference in felt vibration. These things deal with TORSIONAL vibration from the rotating assembly, NOT the vibration you feel in the seat of the pants. That is a balancing problem.
Torsional vibration is what destroys cranks.

Modifying just about anything on an engine almost always has side effects.
It looks like Murray's damper has the advantage of still being able to change fan belts without raising the engine or loosening the spring U-bolts.

The damper's I have used for racing in the past were made from 8.5 inch diameter big block Chev dampers. They weigh 12 lb. In the case of dampers, bigger is better. They have the disadvantage (to some) of having to trim the front cross member to get them in though.

By the way, I never broke a crank when I was using one of those and it was spinning 6000 twice a lap.