I agree that cutting off the weld and bolt-head with an angle grinder may be the easiest way to go, chris. The alternative seems to be to drill axially through the bolt-head and then tap it so you can screw in a stud.
In my view WD40 is a lousy penetrating agent. You might try soaking the nut in Penetrene for a day or two, then clamping the stump of the bolt in the vise and having a try at unscrewing the nut. If you don't get it to move, 20 cycles of heating it to dull red and letting it cool, then another soak, might do it. Other people will have favourite solutions that are worth considering also.
