The rear manifold will have to be separated from the exhaust pipe and
might possibly be able to be removed while it's still attatched to the
head so you can deal with the stuck nut after it's off the car.
I would consider either heating up the nut on the dipstick tube stud
or cutting off the dipstick bracket.
Not the professional way to go about it, but whatever it takes to get
the job done.
This is the kind of thing I was talking about in previous post about
the probability of something breaking, being siezed with rust, etc.
It can turn into a bitch. A torch can be a lifesaver in some
instances. Also one of those pneumatic cutting discs.