Good luck, very difficult to find! Try 318-equipped trucks/vans from the late '60's-early '70's. As you likely know, the majority of 318s came with a cast crank, I don't think any cars had them since they are generally not a hi-performance engine. I think the truck crank has a '318-3' # on it somewhere, maybe someone can corroborate.
I have 2 340 steel cranks, would be willing to part with one, I had to buy both to get 1. They are both std.std, located in N.C. pm me if your interested.