Nick,
When I worked in army workshops and there was a need to drill a hull on a Saracen or Ferret, usually for a mod, we used special armour drills in a slow air drill, to which we put pressure on. You may not be aware but studs are actually welded to the surface, not in to holes, which would be a weakness in the hull. We used a special stud welder. It would save you a lot of grief to go down this route.
Regards, Richard