If you drill it for a "easy out" use a slow RPM, too fast and it will "heat treat" the bolt then you will need a carbide drill bit.
You say it is broke off below the surface, is that the manifold surface or the head surface. If it is the manifold surface remove the rest of the bolts, move the minifold out of the way and use the vise grip method or weld a nut to it.
Here is a link to give ya some options.
http://www.elcovaforums.com/forums/s...ad.php?t=41269
Good luck, Ed