sb crank numbers?

If you are building a numbers matching engine for the car then I wiould think you would want the original cast crank , if it is in good shape.

If you have the original cast 340 balancer then all you would need is the correct 340 external balance converter. Or an aftermarket flexplate that is for a 340 external balance engine or you might be able obtain the external balance weight kit from Mopar that can be used to convert an internal balance converter to external balance.

If you were building a highly modified race engine then I would want a steel crank, but for the typical restoration street performance car I think a stock 340 cast crank would work just fine.

just my $.02,

Bob