How deep did the drill bit go? Sure sounds like you drilled into the water jacket... If that is the case...

I am afraid that your broken off easy-out will not retain the coolant for the foreseeable future...

 

There are some castings that the exhaust manifold bolts are drilled right into the water jacket due to the lack of thickness at that particular spot. Either way, once you repair the bolt a little bit of silicon on the threads will prevent any leaks. As for the broken easy out...yuk! I would try to get a pin point torch in there to heat up the broken part as much as possible, bright red is best...then let it cool on it's own. This will take the hardness out of the broken piece and hopefully allow you to drill it out with a good quality bit. I've done this with Gr.8 bolts but never an eayout...but hey, beats pulling the block. If you have the other piece of the broken easyout, try it on the bench...if after heating and cooling you can remove bits of it with a file it will be soft enough to drill. Good luck. One more thing...ALWAYS use an anti-seize compound on any nuts and bolts associated with the exhaust system...saves all this down the road.