340 Steel Crank Questions

Marine or not the crank still has to attach to something. Never messed with boat motors but I still think the back for the crank should be machined....marine or not.

Yea that's right. You still have to bolt on a flywheel with a starter ring on it. I have messed around with jet boats and they all had flywheels.
I don't see why they can't machine it to fit.