You don't actually need to remove the clutch nut as has been told, but if you do then IMO it best not to use an impact driver. I think the old windy gun is for those that don't have the correct tools, or for F1 where the impact gun probably costs the same as two NSRs.

The impact gun can smash the gear location dogs together which is not really a good thing. The proper tool would be a clutch hub holder. This can either be bought (universal hub holder) which looks and works like a set of mole grips with a couple of long bent fingers on the jaws, or you can make an NSR specific one using a couple of the old steel plates welded together and a bar welded to them. Weld a long bar on and then use a long breaker bar with 27mm socket and you should (reasonably easily) remove the nut.