I finished the extension cord tonight, hooked up the welder and turned it on, and to my surprise I didn't get a fireworks show! Seriously though, I took Wizard's advice and tried reading across the two hot prongs with a meter. Turned out to be a perfect 240v. So I tried hooking it up to the welder, and everything's working great. I made a
page dedicated to it so I wouldn't tie up the forum with pictures. I'll be putting the build of the go-kart and any other projects on that page, too. Thanks for all the help, guys!