340 converter

You have a cast crank....Externally Balanced.
My money is the weight was either not welded back on the converter,or it was put on the wrong place. The weighted converter is a thing of the past. Most people are using a neutral balanced converter (no weights) and using a externally balanced flexplate,along with a externally balanced harmonic balancer.
Cope Racing Transmissions has a 340/904 externally balanced flexplate. $85.00
Team that up with the correct harmonic balancer and neutral balanced converter,and your rolling. Unfortunately the trans has to come out.