Even though the 428 and 391 are both externally balanced there is a difference and enough to cause a problem if they are just swapped. You can have your new flywheel balanced to match the old one. Any good shop should be able to do this. You can even have the 390 flywheel balanced to match if it's still good. I'm pretty sure that crank number is a 391. I'm not positive though. Are you sure it's a cast crank and not forged? That's the confusing part. G.