[Bobboloo]

You still might be able to remove the broken one by welding a nut to it. Since it's shorter it will twist less and is less likely to break. Heat is your friend. Weld a nut to it and then quickly put a wrench on it while it is still hot and give it several quicks taps with a hammer to break the bond. Once it breaks loose your home free.

Be sure to put a torch on the rest of the studs before you try to remove them, (Mapp gas works fine) heat them to dull red and then a few light raps with a hammer on the wrench to break them loose. Tap the stud clockwise and counter clockwise to break it loose. Applying torque to a weakened seized stud is what breaks them. They end up twisting off.