Any ideas on how to free a rusted ball on B&W hitch?

Is your truck a short bed? If so, I would assume the ball is directly over the rear diff. That would be difficult. If long bed, use a floor jack with something like a 6x6 or something of the sort and put lots of upward pressure on it and leave it overnight. It's gonna take heat and/or constant pressure on it along with penetrants. Also, you might want to give B&W's customer service and see what they say. I'm sure they've run into this before. Their customer service is second to none.