RTV as muffler patch?

My old truck muffler has a few holes right where the pipe comes into the case, I cant wrap it so I thought maybe Ultra Copper RTV would fare well enough on the muffler to plug the holes. Would it last or would the muff get too hot? Its about 12" from the cat and another 12" from the Y pipe. The hole are smaller than welding rods, the one crack is about 1" long. Id hit it with my MIG but I bet Ill burn right through that thin wall of rust that I call a 32 year old muffler. Any luck with that muffler patch in a tube?