at least 1/2 inch of threaded stud left after removing the head you could try this:

1) Drill out the center of the cap screw until the head comes off.

2) Heat the remaining stud until it glows red.

3) Grasp the remaining stud with a vise grips and try to turn it a little in and out to loosen it. If it does loosen, gradually increase the arc that you are turning it until in can easily turn out.

The downside of this method is that if you shear off the stud you are back to drilling out the rest of it.