spot weld cutters > step drills
ive got a rotabroach kit for spot welds and it works amazing and is very hard to kill, but the set isnt cheap at all
pick up a reasonably priced spot weld (holesaw style) and use a centerpunch to mark center on each of the spotwelds you have to drill, as that will realy help keep the bit centered as ya work
and remeber, leave the last panel intact, if theres 3 layers its ok if you cut into the second, but once you go thru the 3rd its a pain in the ass to weld things back together