[originalowner]

Not that I've had to do this, but a couple of ideas come to mind. First, apply some heat to the arm at the pivot to see if that breaks the corrosion loose. Second would be to build a puller from a bolt, nut, and some scrap iron for jaws. Weld or braze the jaws to a large nut the size of the arm, slip it over the arm, and run the (admittedly large) bolt in to pull the arm out to be flush with the tip of the post. The puller would be compact enough to get into the space. From there, perhaps the arm will be free enough to get off.