I am going to ask this question in case you can find out. When the motor was rebuilt was the rotating assembly balanced? You will have to know if it is internally or externally balanced. You say Eagle pistons and crank. Eagle sells cranks and rods, if you buy a rotating assembly the pistons are made from someone else. Did he give you any paper work from a machine shop or where he bought the crank, rods, and pistons. To be on the safe side you can buy a ProForm Harmonic Balancer Part No. 66513 from Advance Auto for $80. As for the flexplate was it purchased for an externally balanced 400 cid? It should have a weight welded to it under the ring gear. This is if it is externally balanced. Also is it installed correct. The recessed side bolts to the crank.