Common issue. Mine came out using the largest slot screw driver in my arsenal. Pressing down firmly and turning it with a large crescent wrench (square shaft). If you end up with a welder, consider welding a rod to the end of the stuck bolt. Use something like a cane bolt that has a bend you can use for leverage. My friend 500passwords used an old cub driveshaft. (he shows you exactly how in vid 2 or 3)
Check out his entire restoration video series:
http://www.youtube.com/watch?v=riveN8_5WYo