Of course, with old manifold nuts, particularly on a turbo, the nuts are probably part of the stud by now. I wouldn't even try to get them off with a wrench, unless they look really good.

Apply PB Blaster each day for about a week before removal. IT will smoke a little when the engine's hot but that's ok. When you finally do go remove them, you *MIGHT* get lucky and find they're in good shape (recent work?). More likely they'er little rusted lumps. Buy the new Volvo lock nuts (copper exhaust nuts, actually a little expensive but there's nothing better) and Never-Seize.

Split those old nuts off with a sharp small chisel. This works really nicely. The nut will open up a little and spin right off. You don't need to split it in half, just partway through a side. Avoid hitting the stud threads. Should work quite well.
Good luck!
--
Rob Bareiss, New London CT ::: "Roterande Fläkt Och Drivremmar!"