Balancing a Scat cast crank

I'm building a 408 with a Scat cast stroker crank, Procomp rods, Icon forged pistons in a 360 block. I took the components to the machine shop along with a standard 318/340 harmonic balancer because its supose to be internally balanced to get it all balanced. The shop is telling me that they need to use an external balancer or they will have to put a lot off mallory in the crank to get it balanced. Has anyone else run into this problem?