Ugh.

After disassembly and light cleaning, I would recommend an OA bath for that frame - AFTER you free the post and stem - and there, patience and reserve is your friend. I would make every effort to remove them with time, tapping, and a good penetrating agent. PB Blaster, Kroil or some folks suggest a home-brew ATF concoction which purportedly is even better. "Freeze-off" has worked for some. (Heat works for no one). Excessive force is to be avoided - you can end up bending the stays or seeing them separate from the frame. If worse comes to worse, it's better to sacrifice the components than the frame,but sawing them out should be your last resort.

Good luck - and remember, that bike is worth the effort.