I used to slice the nuts off a bolt with a gas axe way back when I was a kid restoring a Model A.
The other trick I use on a nut and bolt combo, say for a fender attachment, is to tighten them till they snap. Usually works pretty well, and takes less time than trying to loosen with penetrating oil. I've even used that method with captive nuts holding sheet metal parts, then drilled out the stub, but that takes a bit more time in disassembly.
I haven't tried anything other than PB and heat, but have found that even if you get it loose, the nut will tend to bind up and destroy the threads as you turn it off the bolt threads that have been sticking out of the nut for so many years. I have had some success running a die over that part of the bolt to clean up the threads first.
