Welding for the first time - need advice

Actually, even if you overlap the sheet metal without a flanged step it will be ok. If you consider the thickness of the metal it really does not add up to much. I always overlap without the flange step and the use fiberglass with the ground up pieces in it to seal the weld. A little bondo a voila, now one will tell the difference. My bud who restores cars for a living told me this and I have seen his work and it looks flawless. the factory overlapped sheet metal too. Just don't go overboard with the overlap 1 inch will do.
Also another little trick when welding body panels is to have the heat up a little bit higher the what you would expect to use and then turn up the wire speed til it starts to seem like its too fast then turn it down just a notch. the extra wire will cool down the puddle and the higher heat will make good fast penetrating welds. especially when you are just doing one spot weld at a time.