This probibly wont be of much help,but maybe.
Somwhere in 1970 dampers changed to a 4 bolt from 3 bolt. The earlier 3 bolt type shook a little more. I cant find my book with numbers,I do know the 428SCJ motor had a weight between the motor and balancer. Because of the heavy LeMans rods. If the damper looks at all rotted,replace.
Also you may look into other problems,that may me causing the bump. Mounts,tune-up,etc.
Just trying
