A friend was rebuilding his 1980s Muddy Fox a few months ago and we managed to remove / rescue the very stuck aluminium quill stem using plumbing pipe freezer spray, making a wooden plug for the bottom of the steerer to hold a cold puddle in contact (the aluminium contracts more than steel).
You’d hold a good puddle down the bottom of the seat tube where it is really stuck, and remove the bb and cut two discs to make a closed chamber to fill in the middle. You only get one hit at it so prep / seal as much as possible and you need to whack it out quickly when cold.
The spray isn’t cheap and don’t bother with the combined freezer / penetrating sprays – you want maximum cold.