68 dart 360/904 vibration

Did you internally balance the rotating assembly. I would think that would be required with the new rotating assembly. If it is internally balanced you may be mixing standard 360 externally balanced parts with your new internally balanced engine. Balancer, flex plate, convertor should all be neutral balanced with an internally balanced rotating assembly.

That would seem to make the most sense with the limited information provided. Sorry to hear about all the problems with your long distance purchase.