Sleeve the barrel and make a "bunny gun" out of it. Then you can enjoy it for what it is, a mistake that you resurrected. I would never plug, seal, screw or weld holes in a chamber and I have seen this problem before. I have a DB 12ga coach gun that someone tried to cut just in front of the right chamber with a carbide circular saw. The right barrel is getting a nice .44WCF sleeve. Might be an interesting gun when done. We'll see.
No matter what, the gun can be replaced. You cannot!
Just my 2 cents.