Put Argon, Nitrogen, CO2, or even exhaust gas into the tank before cutting or grinding on the screws, and keep it flowing the whole time you're working with them. The inert gas will displace all the oxygen in the tank and make it perfectly safe to use a Dremel tool or something similar to cut a screwdriver slot in the screw heads. Make sure the slot and the blade of the driver you're using are a very close fit. Then try tightening the screw slightly before backing it out.
Jerry
