Just to add to that great post, DON'T scratch the shaft of the new shock, as it will destroy the seal etc. If you need to hold the shaft on the new one, put a piece of leather around it & then use the grips on it.
You can normally loosen the nut with a socket & bar, but then are supposed to use a Stepped Ring spanner & a Shifter to undo the top bolt, but as you may have experienced, if the nut is siezed, it can wind off the holding lug. Good luck
