Broken bolt woes

Ok so I tried drilling the bolt out and turns out that the center of the broken part was not the center of the rest of the bolt (broken part was narrower) So now I'm looking at a crescent shaped remainder of the bolt resting in the bottom of the original hole. What are my options for extracting the bolt now??? This thing is really in there, I tried using vice grips and wound up twisting the broken part off.

There is enough of the ceiling of the original hole left for me to have tapped the hole but I'm worried that it won't have enough clamping force or the broken part would wind up moving... This is the upper left water pump bolt that goes through the timing cover and into the block by the way.

Thanks for the help!
-Dale

red=block
yellow=original hole
black=drilled hole
silver=remainder of broken bolt

If I go a bigger size I'll wind up changing the position of the original hole. As it is right now, I can use a smaller bolt and not worry about not fitting the timing cover back on. Maybe this pic will help explain my situation.

My problem is I can't go with a bigger hole since its off center. I don't know of a way to drill down using the side of the drill to get rid of the rest of the broken part. I need to get rid of the rest of that crescent shaped broken part and I'd be golden but how?

Thanks!
-Dale