68 340 Vibrating idle, loss of power

I'm confused as to what you've got.
You said cast crank (which are factory externally balanced), but you also said '68 block which would have had a forged crank (which is internally balanced from the factory) and then you showed a picture of a 318 balancer which is also for an internally balanced crank.
So, since you say you had it at the machine shop, I'll assume you had the assembly (whatever it actually consists of) balanced? And did you assemble with all the same parts they balanced it with?