[Big hammer]

Snapped off drill bit in a broken stud ain't good, good that you now have asked for help! You need to find a welder / shop who can remove the broken stud without doing more damage. A washer on the block and a nut weld to the remaining stud trick is what is needed. Lawrie's method of removing the remaining studs sounds like a great way of removing the rest of the studs!!! The high intensity heat will break the rust bond and allow the removal of the studs.

When I was working in the airlines we had to remove steel wear strips from aluminum wheels. The steel counter sunk screws couldn't be removed some times and we would have a welder weld 2 towering tits on the screw heads so we could grab the towering tits with pliers a then turn the screws out. This method always work in removing this stuck screws ALWAYS !