Balancing of stroker kits and vibrations

I built my own 408, forge eagle crank, scat I beam rods, srp pistons, balanced by a reputable speed/machine shop. Weighing pistons are easy. After that you need a little more expensive tooling. They balance rods one end at a time. Bob weight is determined by the weight of pistons and rods on each journal. Factory 360 bob weight is about 2150 and is why it's externally balanced. Cast cranks are to light and must have Mallory metal added unless the bob weight can be lightened. Stroker pistons make it easier for internal balance because bob weight is much lighter. After market rods are usually lighter too. Your problem is probably in improper balanced crank or bad harmonic balancer. The crank can probably be repaired. No expert but I did a lot of home work when I built my motor. I didn't have the money to build it the first, sure didn't want to build it twice. Engine bearings always tell the story. Hope this helps, Russ