you need to drill them out. Center punch the bolts and drill a 3/8" hole thru the bolts and then use a 3/8" bit to drill just till the heads are drilled through and chizer the heads off. Cover the gears so no chips get in and use a 1/4" drill do drill through the remaining bolt shanks and use an easy out to remove them. If they still will not come out try opening the hole up a little more and retap the holes.