If the shaft is really badly damaged, the drill & tap repair that's been described might be your only option. Is the shaft hollow, with the horn wire coming up the middle? If so, you've already got a well-centered hole to guide a drill bit, so you can enlarge the hole in the top of the shaft 1/16" or so at a time until you can thread the hole for a 7/16-20 bolt. That tap drill size for that is about 13/32", or a "W" letter drill. It will probably be necessary to hacksaw or grind the damaged end off the steering shaft to get a good drill/tap procedure done on the end of the shaft.
Once the steering shaft is drilled and tapped, it will probably be possible to center-drill the bolt and run the horn wire back through the middle of it. Another option would be to use an aftermarket horn button clamped to the side of the steering column.
Jerry
