Maybe you can find some sort of cap on there? To hold in penetrating lube? Fill it up nd let it soak for a couple of days.
Ideally you have on oxyacetelene torch and a 1" impact that will separate and vibrate it into submission. Otherwise you could get a functional propane torch, heat up the outside of the nut, and hit it with the biggest wrench and cheater pipe you can find. Like 6+ ft. long cheater pipe (that thing is rusted to hell).