If you can't get a visegrip plier on it, you could try tightening the nuts from underneath until the bolt breaks. (last resort)
Or, if you have a dremmel tool, you could try cutting a slot in the bolt head so you can use a flat blade screwdriver.
Or, if you have a grinder, you could grind off the bolt heads, or if access is easier from below, grind/cut off the nuts.
1974 Boler 1300
Manufactured in Winnipeg, Manitoba.