a518 Vibration

Could it be RPM related since in lock-up the RPM's drop? Have you tried it in neutral at different RPM's.

We recently had a 360 that was neutral balanced when rebuilt with the external balance damper. So thinking it was an external balanced engine and putting it in a 4 speed car. We used an external balanced flywheel. It vibrated bad. We installed a neutral balance flywheel and the vibration was cured.

Many times when you have an engine rebuilt with new pistons the shop will balance it with the parts that you give them. If they do you now have to use a neutral balance torque converter.

Just a thought you may want to look into since a 400 is externally balanced from the factory. Was it rebuilt at a machine shop at any time since new ?
Usually when this is done you will see a heavy metal plug in the rear crankshaft counter weight.