Sorry if I seemed grumpy - certainly didn't mean to! I don't have a lot of different ideas on this that look fool-proof.
Rick's right, of course, and that occurred to me on the way back from a Dunkies run: If I did the welding using the bolt as a guide, I sure as heck would weld the two together or overlapped the thread area, especially if I MIG'd it. Don't be like ME! The brass or wooden plug sounds good as a shield, though. Whatever you can find that works, BUT I would also find a well-recommended welder in your area, tell him what you're trying to do and see what he can suggest. You can't be the first person trying to restore something like this so he may have some good ideas, too.
If I think of something else that makes sense, I'll be back.