rhythmic vibration

Listen........Been there, done this a number of times...........

Chances are good that you're looking at a mismatched balance between the motor and 2 components..........

The 340 you mention should be internally balanced.......There may be a counterweight bolted to the inside of the harmonic balancer up front....If there IS a counterweight, you need to remove it asap........It will cause exactly what you describe......

Secondly, some guys swap out their torque converters too for a higher stall.......if they ordered an externally balanced converter, this will also add to the rhythmic vibration. You need to remove the weight (welded) from the converter, or replace the converter with a neutral balanced unit.

Stop throwing money at it and check that balancer FIRST......then check the converter. :read2: