Seeing how you got that bolt so tight that it snapped the head off, you may have overloaded the bolt and hole threads and what's left of the bolt might be really hard to get out.

So before you start drilling, find you a flat, round punch just small enuff to ease in the hole and not touch the threads. Then smack the bolt hard with a medium sized hammer. That will unload the bolt and hole threads making it easier to get out. Don't use a pointed punch or it will flare the bolt and make it even tighter.