JE, from what I am seeing in your pics, Elkhorne is spot on. Unfortunately you have a gap where a tight fit is critical. The breach end takes all the recoil and should have no gap. I would cut a piece of wood with the proper grain orientation(end grain) to fit in the "gap" area leaving it a bit tall or proud and epoxy in place. Once cured, file,sand, scrape to blend. Getting a piece from the stock you are using would be ideal, if not, I would try to hunt down a piece of Walnut and use that. Like Elkhorne mentioned, the grain orientation will be critical in this area to assure strength.