There is lots of info in this thread.
Last winter I saturated the four mast step screws with WD-40 (screws all sitting in a pool of WD-40). In the spring I heated the screws with a propane torch and smacked them with an impact driver. Screws came loose
My biggest worry was that I would set the boat on fire when I ignited the screws saturated in WD-40 (I did not):)
I, of course cannot assure you that you won't break the screw heads off, but this technique worked for me