I echo Mike D on the drilling off center into the steering box casing story... also personal unfortunate experience. This is why I was suggesting that if the remain of the bolt is protruding enough out of the steering box, you may try to weld something on it to undo it. You may take a piece of flat iron, drill one end of it to the diametre of the bolt, put the hole on top of the bolt and weld through the hole. Careful not to weld everything together with the casing...

If you have no option but drilling, make sure to use a column shaft driller and that your steering box is VERY WELL tighten to the chassis of the column shaft driller. Use high quality drill and do not put too much pressure as this is when the whole thing is going to go sideways...