Is the hole off-centered far enough to damage the threads yet? If so, it's going to require some heroic measures to correct. You might try bolting an old pump housing onto the block and using the hole in the pump as a drill guide to try to remove the rest of the bolt. That's not likely to succeed, as the drill bit will probably try to follow the hole that's already there and just hog out the pump housing. If I had the block at my shop, I'd use the water pump gasket as a pattern to make a 1/2" steel plate with the pump bolt pattern drilled into it, and open up the hole in question big enough to accept a hardened steel drill guide bushing. This is bolted onto the block and used as a guide to drill out the remainder of the bolt, and then heli-coil the hole. This procedure would involve about $200.00 worth of fabrication work! It's a lot simpler to get a pilot hole exactly on center in the broken-off bolt and avoid the off-center problem in the first place.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!