Hatchet-Jack
54 Cal.
You can close the gap by cold smithing the metal to stretch and reshape it, then file any hammer marks flat. Don't do this unless you're confident that you can hammer and shape metal.I don't suppose there's a way to lessen the gap where Pedersoli inlet it too big, is there? Probably not without fudging things I imagine.
The other way is to fill the gap with a stainable epoxy. Apply a thin layer of paste wax on the brass so it will not adhere to the epoxy. With the side plate in, fill the gap with the glue. File and sand it flush.
Good luck!