[Lynn Eades]

Some options;
Use an impact driver. Give the head of the screws a hit with a flat faced punch, (square on)having carefully supported the casting at the other end of the screw. This is to jar it free. (do this first) Judiciously use a blunt cold chisel to chase the screw head in an anticlockwise direction. (one well judged blow)
Carefully heat the screws and let them cool. If that doesn"t free them, heat them again, and quench them with water.
One or a combination of the above should free them.