Alot of the silver/tin alloy soft solder around melts in the 400F range and will work well but I agree that plain old lead/tin worked fine in the original application and no reason to look elsewhere for a fixer. Alot of the stuff around now is 'lead free' silver alloy soft solder and it's marketed as Silver Solder though it's not the silver solder I think of. Plus the silver alloy soft solder stuff will stay forever bright white and never tone down to the medium grey of the lead/tin stuff if thats a concern on an older gun for looks. It'll always look like a repair as will an epoxy plug, but they'll work OK of course. Using the older zinc chloride flux will get you a great solder bond even with a less than perfectly clean prep surface but it will almost certainly guarantee some very active after rust in there if the stuff isn't completely flushed out. Another step to add to what starts as a simple resoldering job. Clean the edges of the surfaces well, flux with a noncorrosive flux and a touch of solder with your favorite applicator (electric or torch) and you're done. I'd wire and/or clamp if using a torch (which I don't use anymore) as alot of heat is dispersed into the barrels and ribs even with a small flame. No need to, as I said, if using a soldering gun or iron. If it takes more than 30 seconds to do the actual soldering at the muzzle, you're doing something wrong.
If you want to add a triangle steel piece to the opening, take a piece of round stock big enough in diameter that you can file to a three sided, tapered spike that will not fall into the opening but the three sides will hang up on the rib and barrel edges. It doesn't have to be overly long inside (watch the length on top that it doesn't interfere with the front sight bead). Tin it, clip it to length so there is just a bit of extra to the outside, drop it into place, solder and face it off to the muzzles with the soldering job.
FWIW, the soldering iron or gun is perfect for tinning the ribs and barrels for relaying too.
