oil pan gasket

If they have welded an extra plate/shroud over the oil pan even jacking the engine up won't let you get the pan out most likely. Sounds like your either going to have to cut off the plate they welded on to get the pan off on the car or yank the engine. I can tell you changing the oil pan gasket can be done in the car but it's not easy at all. Of course with the engine out and turned upside down it a breeze.

Did you try a long 1/4" drive wobble extension with the appropriate socket. Wobble extensions make getting into hard areas a lot easier. They can be bought at Sears.