Personally, I think that you would be much better off grinding off the old weld completely and re-weld the crack.That doesn't mean that another spot weld won't fix it but chances are that you won't have weld that is as reliable as a whole new weld. Being that you don't know how clean they got the metal to start with before they welded it. I would also get a small cutting wheel like the ones used with a Dremel Tool and open up the crack a little to aid in weld penetration without creating excessive heat and possibly having problems with distortion. This will also give you a cleaner weld, making it much stronger. Generally speaking; a weld is stronger than the metal surrounding the weld if welded properly.