I think I need a special tool to remove the nut and bolt that hold the exhaust manifold to the catlytic converter. The nut is square on the base and then round on the to top. The square part is not welded to the base as far as I can determine.

When I try to turn it from the bottom, ie the 13mm bolt I have no success. Since the threads go into the square nut I think I would have better success applying force from that end, but I need to get a complete and very good grip or I'm going to make a mess.

Would it be better just to cut the bolt and replace? I can cut the one in the photo with a dremel but don't really have access to the one on the other side.