If you don't mind trashing the post I've had luck with a vice grips and dead blow hammer. The first step is always to use a penetrant like PB Blaster and give it time to work. Then heat helps. I think the heat helps soften the rust bond more than actually expand the metal. I use a propane torch for heat and have never damaged the paint but YMMV. I like the hammer/vice grip method since the force is nearly all torque on the post. Plus it's very satisfying to hit the post over and over until it moves
Once it starts moving keep going around in a spiral motion until it's free.