I think the stock cast cranks are intended to be externally balanced, the 4340 aftermarket forged cranks are intended for internal balance. The size and weight of the counterweights on the cranks are different. You can probably get your cast crank internally balanced, if the balance shop is good, but the mallory (heavy metal) necessary to balance it will probably triple the balance bill.
Cheaper to use 5.9 accessories with external balance, than spend lots of money to internally balance just to use 318 parts.
Lighter internal parts will make it easier to balance, because that means weight is removed from crank counterweights.
