Boiling water can help, especially if you can actually boil the parts. If the aluminum surrounds the steel chances are much better. If you get it to move at all then rock it back and forth to help eject the corrosion from the gap. If it moves then 50/50 fuel oil and motor oil seems to work as well as anything to keep it moving. When you put it back together don't forget to put anti-sieze on everything.
Bill

I remember an old trick about using wax. ( actually I only kind of remember it) I believe that when you want to remove steel or stainless bolts from alumimun where galvanic corrosion has set in, light up a standard white candle (for some reason it has to be plain white, unscented, etc) and drip some hot wax on the threads that are exposed. apparently the wax will somehow penetrate the corrosion and break it free. I haven't tried this, but as I said, I read it somewhere.. I think it was an old issue of Hotrod magazine tips and tricks. the guy that sent it in was trying to remove bolts from an aluminum head that were seated pretty good.
not sure if it will work, but if you try everything else with no luck, what is there to lose?
good luck