I can get a wrench on the nut but, an open end wrench doesn't have enough contact points to not round off the nut. Hope that makes sense. I may be able to "tap" a box wrench over the nut that would allow enough grip without further damaging the nut. I'm just not sure how much force will be required to overcome the "staked" section of the nut that is in that valve stem groove and if the valve spring retainer portion that indexes into this groove would be enough to stop the entire valve from rotating.

I could always chase the threads on the valve stem after the nut is off to clean up the threads if need be but, then the question would be, how do I hold the valve from spining as I run the die down over the threads? Maybe a thick piece of rubber and a pair of vice grips? I definitely don't want to damage the valve stem itself.

__________________
1982 300SD
1992 190e 2.3 8v
2002 P38 Range Rover