Man, that stinks.

I suspect a tang is going to be hard to come by. He screwed up a simple job and should make it right by either welding it up and drilling it correctly or replacing it. Didn't you say he had some TCs leaning against the wall?

EDIT: Another option (if your's satisfied with how the tang looks) would be for him to provide you a higher front sight.

You might want to try this trick to see how high a front sight would be needed, and whether you can live with it. http://www.huntingnet.com/forum/blac...-how-high.html