welded drivshaft in the middle, YES that is your vibration some people should learn how to do things right, not just do whatever they think might work

good luck on the new engine purchase

what are your plans for the frontend and brakes?

hope you dont get discouraged with your project,
there are some good guys here that will help you through the issues the last owner made for you.
just ask and we will try to keep things moving for you Ron....