This is a tricky situation. You may need to replace some parts here. I was first going to suggest penetrating oil (obviously), but you already used that. Then, I was going to suggest you apply some heat from a small torch in there, but you said there wasn't much room. If you could get one of those tiny torches in the area, you could try heating the nut (not the line) until it's almost red hot. Then, let it cool and try the same thing again. Attempt to turn the nut with a wrench to see if it breaks the rust at all. After all that, put a few drops of motor oil onto the threads and, because of the heat, the oil should get sucked in like solder. Keep working the nut until it's loose.
Other than that, you could try tightening the nut first and then loosening it, but really, you may have to cut some lines and start over. If you do end up replacing the lines and the fitting, be sure to use a rust inhibitor on them, such as Fluid Film. This stuff doesn't affect rubber, so it's safe. You'll need to get your car or truck treated with it once per year though.
