I'm not a welder but I have used a welder who has a laser micro-welder. He could plug the barrel holes without melting the solder. I have used lead shot and cold blue will darken the lead.

For the butt, cut off the swivel and make the base into a screw with a slot. Polish it and fire blue it and it will look nice.

Now I just take out the swivels, drill the holes out to QD swivel size and use a sling when I need it.