broken stud removal

I have a block drain that is rounded off an giving me fits to remove. The plug now is like a bolt with the head broken off. I tried welding a nut to it, but the weld does not penetrate the plug. It doesnt appear to be a brass piece, as it is not very soft. Is there a tool that "grabs" exposed studs to rotate it out? I havent had any luck with vice grips and heating the block up with a torch...especially since the engine is in the car. I've been soaking it the past couple days with Gibbs penetrant.

Thanks,
Tom