Small block Chevy's are internally balanced, and big block Chevy's are externally balanced (they have the weight on the flex plate). I had this problem a number of years ago. Got a small block flex plate behind my 454. The mechanics noticed "something" was wrong but didn't know what. I had built the engine and had laid the crank flat on the ground for sometime. I was worried I had warped the crank. I talked with a master engine builder later and he asked me if I had the "weight" on the flex plate. I checked and it wasn't there ...ie wrong plate! Damn thing vibrated my water pump off! *G* That was fun!
400's are kind of an odd ball engine, they are configured after a small big, but are externally balanced like big blocks. NOW I'm not a 100% on this advise, but it's a starting point for you to check on. GOOD LUCK! ...Mark