I think I heard it from rigger Brion Toss, warning about failures in rigging attachments if the clevis pin is undersized. The problem is that it puts huge point loads on the items being joined. I would think that the same risks could apply to chainplate attachments. With multiple bolts to spread the load to the hull/knees, I'd guess that stainless chainplate probably wouldn't fail. That's for the engineers to calculate. If you can find 9/16", might as well use them, just to be safe. Rob are you there to comment?