wha kind of metal drill bit would work best to drill out a head bolt.

one of my head bolts snapped while installing!

Do you have any material above the block?? If so, I would weld another bolt on top. Use some PB Blaster and let it sit for a while. Also, might use some heat. If you have no material above the block, then you have to start drilling it out. If you have quite a bit of material above the block, you might try pounding an impact socket on it to try and remove it. Or, you could grind it squire and use a 12-point socket.

If possible, take the block into a machine shop and let them do it - get a quote first.

Be very careful about using any EZ Outs because if they break, you are about totally screwed as they are hardened steel and cannot be drilled out - ask me why I know?

If you decide to drill it out, use a good, carbide tipped drill and try to start drilling right in the center - use a punch to get a starting point. Use larger and larger drills until you can get it very thin. You will most likely screw up the threads, so plan on a helicoil job.

It is much preferrable to take it to a machine shop - they are equipped for these issues.

Good luck whatever you decide.