Check your serial number. ’56 was the year OMC made mid-year changes and the RD-Big Twin shear/drive pins were among the changes. I believe this is when they started using rubber hubs in the prop and a stainless drive pin to match it. The earlier models used a bronze shear pin.
Here’s a snip from a Johnson book explaining to use the later 304575 drive pin with the earlier props. If you don’t want to do that, a piece of brass rod would work.
