Hi Floyd;
just do what you have to to cut, mill, Dremel, or whatever the rest of the bolt head off, then providing that you have the other head bolts out, the head will come off, then you can grab what's left of the broken screw with your favorite pliers or vice grips. If the leftovers are reluctant, apply PB-Blaster and a monokote heat gun and it will eventually come loose.
Type at you later,
Dan McEntee