While you are waiting on the PB Blaster did use a good center punch on the frozen screw to give you drill bit a good starting point. This would keep it centered and away from box casting. Also start the bit slower RPM till it gets some depth into the screw you do not want to over heat the bit.
Don't use regular drill bits use the pilot bits. They won't walk around I can't find them now I always forget exactly what they're called. Hopefully Bruce will put the link up for them again