Here's a tip I just discovered the other day on removing rusted underbody and exhaust nuts, especially in places where you can risk stripping or snapping off a stud:
After applying PB Blaster liberally and letting it sit (twice a day for a couple days if it's really bad), clean up the bolt. Then,
slather the end of the stud/bolt in anti-seize.
Most of the badly corroded nuts I've had gall up have done so *after* they start running over the rusty portion of the thread. Clean steel typically won't gall by itself, but rust will. Letting the nut run over the anti-seize instead of the dry rust seems to help a lot.
If you've already seized up the nut badly, pretty much the only way to get it off is to either heat it off with a torch, or cut it off with a nut-splitter like
this one (be careful not to damage the stud though)