Heat the bolt, not the surrounding metal, with a jeweler's torch until it starts to glow. Try to both loosen and tighten it.
If by some miracle that fails to work, drill the bolts down the middle with increasingly larger bits until just before you hit the threads. Then you can crack it out in pieces and run a tap through it.
And if THAT doesn't work, cut the whole door jamb out and replace it with a donor one from another cab that's not so rusty.
