Bad vibes.

Have a problem with my friends 360. It has a bad vibration all the time. Its a stock rebuild with stock rods, pistons and crank all from this motors. Has new rings and bearings. Rebuilt stock heads, comp xe262 cam lifter and springs and new timing chain. It runs beautiful, doesn't miss a beat. When we put it together it had a after market 360 balancer, stock flex plate and weighted converter off the old engine that didn't run when he bought it. The balancer looked just like a stock one. Cut out and all. Converter looked weighted right but it vibrated. So I had a nice stock balancer so we put it on just to be sure. No difference. So next we ordered the correct b and m flexplate and used a neutral balance converter, again no difference. I'm stumped. We checked everything. Took off accessories, made sure nothing is wedged or touching. Does it in neutral or while driving. Could it be the tranny, it works fine. Any suggestions would be appreciated. Thanks.