I've spent many hours seam welding my Z using a Millermatic 180 with the smaller .024 wire. I'd strongly suggest the smaller wire as it takes less voltage/heat to melt it. As for the machine settings, stick with what your machine says on it as it will give you a pretty good setting to start at. It never hurts to practice on some scrap metal of a similar thickness first. For thin metal, don't weld for more than a second or two at a time or you'll blow through. Really the only places I had any trouble with blow through was where the metal had been stretched thin by presses during the manufacturing process. If you do blow through, be patient, you can fill it back in little by little if you give it time to cool, then grind it back down to how it's supposed to look.
