Stuck exhaust manifold nuts can be a real nightmare.
Start by spraying it very liberally with a penetrant, and let it soak
overnight.
I use PB Blaster.
Next, you need to be able to get a wrench on it.
Do you have a ratchet with extension and one of those swivels to put
on the end?
If you can get an angle on it, that is the way to go, otherwise is the
closed end of a hand wrench, preferably one with a slight angle to it,
and work it off.
Just be patient: work on cars is one endless process of problem
solving.
If all else fails, perhaps you could chisel the nut off, provided you
can get the correct angle.
