If filling holes previously drilled to open the safe you really ought to be thinking first of the hardness of the material being used.
The best choice would be the slightly tapered carbide plugs available as these are made for the job. Failing that a ballbearing welded in situ is good. If the door will stand tapping you can always tap it out and insert a machine bolt. I like Allen bolts as they are higher carbon as a rule so can be hardened batter. If you go that route, ensure that you do something to prevent the bolt being unscrewed. That can be as simple as putting the bolt through the door with a couple of threads protruding and peining those over.
