Oh, I forgot to mention;
Sometimes if you drill it out with a left-hand bit, it will break loose during the drilling (before you get all the way through). Less drilling this way. Use largest bit that will not damage thread. If this works, you will not have to go oversize.
Permatex copper antiseize may prevent the next fastener from breaking, though I try to switch these sorts of things to stainless if I can get a match.
Corrosion is accelerated dramatically at these temperatures. Mix that with our roadsalt, and the heads on these fasteners often dissolve away.
1986 300E 5-Speed 240k mi.