If the machine shop said you have to use the notched flex plate then you should have a matching cast crank balancer in front. Plus, I assume you have a cast crank to match the notched flex plate, and the machine shop knows this, even if you don't.
You must have 3 matching rotating pieces, all for a steel crank or all for a cast crank. You had 2 steel crank pieces on it, but if the crank is cast, the parts still didn't match.
I don't know who the machine shop is, but just because you gave them a steel crank to use, doesn't mean that's what you have now.