I got to wondering after reading the whole post again if maybe someone used LockTite or some other form of gorilla snot on that nut. If so, you'll have to heat the nut to around 400 degrees to break down whatever is in there. After you get it off and see a whitish powder residue on the threads, that's what it was. Clean it up well and oil the threads before putting the nut back on. .it won't reactivate but there's no sense in leaving it on.

Hello ol'Grump, You are probably on to something there. I'll heat the c... out of the nut and see what happens. Again, thanks for the advice. Joe:D

Well, I finally had success in removing the nut. I had to heat it up good with a map gas torch and gave it a good wrenching with a 3/4 drive breaker bar and pipe extension. They had used silicon instead of the required gasket. I installed the cork gasket and put some permetex gasket sealer on the lock ring and tightened everthing up. No leaks so far.

Glad ya finally got it Dozerman. That's got to be about the worst one I've heard of. Usually a couple'a smacks on a slug wrench and they loosen right up. I'd say someone tightened the dickens out of it last time. A LOT tighter'n it had to be!