The best bet may be to get a heavy duty cutting wheel on a Dremel and slice that nut off there. I do an X cut across the nut and bolt, the take a hardened chisel and bust it off there. Does not take long at all. You can buy a new metric bolt and nuts at places like Lowes and True Value Hardware.

You do know about the hidden nut on the top back side of the compressor? That was my major did not know, well that and the fact that you just tighten the 10mm nut to tighten the belts. Feel along the top back side of the compressor for an oval headed nut. On the back side of that is a 13mm nut that you need to loosen but not take off. You loosen all the 13mm nuts that go through the accessory bushings, then the hidden oval nut. You do not need to remove anything. Then you crank CCW on the long 10mm nut at least an inch or more, and push the alt in towards the block. You tighten the 10mm nut until the belt is tight before you put a wrench on any of the 13mm bolts and nuts when putting it back one. That works for the power steering pump and the two on the Alt also.

Good Luck,

Paul