I have a couple of questions to ask you.
Are you using the motor out of the 71 3004bbl440 HP
Second question: did you have the motor rebuilt and put to neutral balance.
If you didn't71 4bbl 440 HP's are externally balanced 6 pack rods vibration damper, you need to use a 6 pack fly wheel, not one off of a neutral balance motor,

Plymouth's gonna getcha! There are 3 things that will live forever, cockroaches, Keith Richards, & slant 6 Dusters 71 gtx 440 4speed air grabber blue on blue with white vinyl roof