HELP 408 stroker bad vibration

I'll jump in here and make a guess. I would think that if you are using the torque converter from the original 318 ,unless you are using the special B&M flexplate, that could be your problem. I know that torque convertors are different for externally balanced engines like the 360. 318's are internally balanced.

I'm sure someone who knows more will jump in here soon with us!