What you need is some good penetrating oil, I use a product called 'Gibbs', but others may work. Even veterinary iodine. Put some around the broken bolt, then take a flat punch and hammer, on the center of the bolt give it a few good smacks, to set up a vibration, then put some more to soak in, a few hours, or days later, repeat, do this several times. NAPA sells an extractor that will work on this, kind of pricey, or you might have a pipe wrench, about 6" or so, small enough to get a good bite, and a cheater pipe a foot long or so. Another option is if there is not enough stucking out to get ahold of, cut a slot across the center of the bolt, like the head on a screw for a flat tip screwdriver, you can then use an impact driver, like they use on motorcycles to tighten case covers, it will come with a bit for large flathead screws. Try turning both ways, if it will budge one way, it can be pursuaded to go the other way also, most times. Eventually it will come out, if not get in touch with me, there is a way to minimize the possibility of cracking the block when heating, but, it is somewhat involved. It is better to use this as last resort, and it sounds like you still might have options! 
Have a great day!!!!
Andy