[HardcoreZ28]

Welding a nut on is still your best bet. Try to find a nut that will fit over what's left hanging out of the bolt. You need to really weld it good...burn it in there. Try to focus the heat on the broken stuff and then pool the weld onto the nut. You'll probably distort the nut but you can use an adjustable or even vice grips to loosen it after. Also the heat from welding should really help loosen up any rust in there.