I had similar troubles with mine and did my best to patient with lots of Kroil over about a week, still no go. At that point I went the Geo Hahn route both spreading the clamp open a bit and unbolting it from the body work so I could give it a bit of a twist to try and break things free. I then reattached the brackets to the body work, put the sacrificial nut on there and worked at it some more. It finally came out, but only after the introduction of several new (or at least new to me) profanity laced word combinations.
Once it came out, everything got a nice clean up to remove corrosion, fresh paint and everything slathered with anti-seize on installation. If I ever have to yank that thing off again, it had better be easier or else I'll be adding lots of stuff to the profane pantheon.