So I have this rescued LeFever Nitro Special I've been working on, PO said it was having light primer strikes and firing both barrels when it shouldn't. Tore it down for a clean'n'lube. Quite the pain to put some of those springs back in, but it feels a lot smoother now. Put on a new bead too. Still need to test fire it. The barrels had been cut down to 18.5"(yes I checked) with a hacksaw, and wasn't cleaned up after. I've been working on trying to even up the muzzle face slowly with a file while trying to keep as much metal as possible, so far just taken down a few high spots. When I finish up that part, there is a gap between the barrels. I'm wondering what the best way to plug that hole is. I've never handled a double barrel shotgun before, so I don't have much to go on, I would imagine that there is usually some sort of filler material soldered in there on a normally finished gun. What would the preferred method be? What's going to be the easiest and cheapest way to finish up a $100 shotgun?
Pic of muzzle:




Pic of annoying top lever spring(with the threaded peg behind it) and dremeled Harbor Freight clamp to reinstall it:






 

Fire....fix it with fire.

Any cut off SxS is going to have the opening visible, doesn't hurt anything to be there.

You can fill it in with JB Weld, Accu-Glas bedding compound with its black dye, or file down a  piece of metal to a close fit and JB it into place.

I used the Acraglas gel with the black die to fill mine.  Make sure you use a combination square when truing up the barrels.  It takes a few hours with a file, but the results are worth your time.