Since the only purpose of the threads and nut, is to keep the steering wheel from falling off, not to engage the shaft, one can remove the whole works, put it in a lathe, turn new threads, or, drill out the shaft for a smaller bolt to hold the steering shaft on, or,
carefully, drill and tap the shaft in place for a smaller bolt, leaving enough meat between the splines and the OD of the new bolt.
Only do this if one is skilled at this, otherwise, get someone who is.
I did this as an emergency repair once while in the service, used a 1/4' drill and hand taps.
