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1-PBlaster, applied 2x daily for 2 days. It is NOT just an "penetrating oil" it chemically changes corrosion products.
2-CO2 or other "freezing" instead of heat. No chance of detempering the metal. Ice it down with a CO2 bottle (which will cost about $25 to refill) or pack it with a rag full of dry ice slush, made by chipping dry ice into alcohol. Let it ice down well, them pull the ice off and use the impact wrench. Repeat 3x if needed--I've never needed more than that.
There are some "freeze sprays" you can buy in electronics stores and other places for similar purposes, they are just not going to give you as much cold for the buck. And unlike liquid nitrogen (which you can buy from industrial gas suppliers too, just bring your own thermos) the CO2 should not be as dangerous to work with, or shatter the metals.
Remember, dry ice can give you frostbite, "safe" is a relative term.