Oh crap..... Internally balanced engine with an external balance flex plate?!

When you balance a crank, if you use a neutral weighted harmonic balancer and a neutral weighted flex plate/fly wheel, you don't spin them, it just adds more weight to mess with. It's only if the assembly is externally balanced do you spin the entire mess.

I would simply call the shop and ask how they balanced it and then use the suitable flexplate, which will probably be returning the external flex and getting a neutral flex. S/F.....Ken M