I had the same problem below the driver's floor pan right my the oxygen sensor, after using Liquid Wrench and tapping it a few times, it came out using a 6 sided socket on a universal. The bolts were trashed, and I ended up replacing them with new ones. Inspect the area of the exhaust gasket, if it is slightly warped you may end up replacing that gasket again. The space was cramped, and it seemed like it took an hour to get the bolt on the engine side in as I could only get about a quarter turn each time....
Hope that helps,
Boot