I've had a few 391's break cranks, but it always happens after the bearings go away, so she's screwed anyway! 318 trucks do the same thing, where a cast crank will rattle on for weeks with loose bearings a steel one will break. Had it happen on diesels, too.
By the way, if you are going to use a truck block, bear in mind that the pilot bearing at the bottom of the distributor, where the oil pump drive engages it, is bigger than the car ones. Either bush it down, or use the truck dist. and pump and drive. It's 33/64",I think.