I have seen many cranks broken as yours is. All of them were externally balanced.
Some were traced to cheap harmonic balancers, some from loose balancer bolts and SEVERAL were the early design SCAT LS stroker cranks that needed 3 or 4 slugs of mallory metal in each end counterweight to achieve "proper" balance. Scat has finally changed all of the counterweights on the LS cranks, so they will internally balance like everybody elses units.
My new build uses a Scat crank that can be internally balanced, but I had to remove a TON of weight to achieve zero balance prolly because of the Wiseco pistons lighter weight. A lot of the weight savings is from a more normal weight piston pin. I believe anyone building an engine with stock replacement pistons buy a lighter set of pins, as stock ones are waaaay heavier than aftermarket ones. A better alternative is lighter aftermarket pistons anyway.
If you bring a Scat crank in to be zero balanced, expect to pay extra for the time for all of the weight that needs to be removed. I think its worth it in improved performance. Also, dont forget to balance the cast iron pulley spacer deal too, mine was way off, even tho it was factory balanced. It does hang way in front of the balancer, putting more load on the front of the crank.
P.S. I also zero balanced my stock flexplate and balancer rather than buying aftermarket, as mine is a "street car."