x2 and if you use a little thicker tin like you mentioned it'll be easier to weld.On a street car, stitch weld it along the edges of the tin. Use silicon bronze if possible, and always lay out your weld pattern so that you can wrap the corners at least 1/4" in both directions.
Not sure I follow what you are saying. Could you post a pic?If you will bend down a (about) 3/4" lip at the edge of the panels it hides all the welds. Example, the panel between the tubs....bend the edges down on the outside (tub side) of frame and tack weld. Also, anywhere two panels join together...do them the same. Makes for stiffer panels and unseen welds.
That's it. Nothing wrong with tack welds, rivets or screws showing....in some cases you can't hide them. As I said about the panel between the tubs,instead of having the welds or rivets on top of the frame... as you explaned (better than I did) make the bend and fasten it to the side of the frame.I think he is saying that instead of following the rad of the round tube, you go to the od, then bend 90*. The adjacent panel would have the same 90* bend (putting the edges out of site) then you spot weld or stitch weld the two edges together so the actual joint is hidden from view.