I'm currently restoring a 50 year old truck, and every bolt on it is rusted or rounded off. Craftsman makes a spring loaded camming-type wrench that I have had good luck with. As long as there is some reasonably flat surface remaining, it seems to get a pretty good bite. You just have to be patient and apply pressure VERY gradually, or you're asking for busted knuckles. Needle-nose Vise grips clamped on really really tight (so that they actually bite into to bolt a bit) should work too. You could also try cutting a slot in the head with a Dremel tool cut-off wheel, and get a flat blade screwdriver attachment for your 3/8" socket wrench. Put that in the slot and break it out that way. Method three I almost guarantee will work if done correctly, but it is a lot more work than the first two, and a little more dangerous in those close confines.
Good luck. I feel your pain.
