The threads do not matter, use a nut that is one size to big. If you have a 1/4" sticking up it should be enough.
If not then do what Gary said. Just be very carefull to keep it centered! Buy sharp new drill bits and GO SLOW! I would start by grinding the top of the bolt flat to be sure you can punch it exactly in the correct spot.
Blaster is common now at most parts stores. It is a penetrating oil. It is the best, settle on nothing else.
[This message has been edited by Electrathon (edited 11-10-2003).]