Also, I bought a 13/16 wrench just a hair to small....set crescent wrench to 22mm...seems a hair to big and just barely rounded a corner on the nut.

 

Not sure what size it is, but.... chemicals are going to be your best bet. Exhaust gases flow thru there, which are a bit corrosive.... so, the fittings are going to be rusted together quite nicely. Hit it with some chems, and then tap on it with a hammer.