While I am certainly not one that advocates welding guns, in THIS PARTICULAR CASE your crack is NOT in a very critical spot and even if it cracked all the way through it would not go above the pin hole. I am not all that knowledgable about welding alloy but a Master Welder would be and he could certainly advise you about possibly doing so.

If you did nothing and shoot the gun you might have the crack spread and a piece come off - but again it would not go to the other side if the hole so I don't see a huge safety issue - just a possible loose grip issue.

Since you can't get it replaced by S&W and can't in good conscience sell it like that, I see little downside about talking to a professional welder. Nothing to loose.