Broke a bolt, any suggestions?

I just had a similar problem with broken bolts in my V10 truck engine. I was able to mig weld a nut on the broken stud, let it cool a minute, then used a wrench/socket to fit the nut & remove the broken studs that way. It worked like a charm, & the broken studs were in the aluminum thermostat housing that was seized in place from galvanic corrosion. May work for you if you have the room.