Nice truck.. The copper is a good tip and they actually make backing tools that are copper to pull the heat away. I got to where I felt I could TIG decently and then tried some sheet metal.. I found out really fast that it is entirely harder to do without burning a hole. Also I would go with thinner wire than 1/16 if possible. It seemed like the sheet metal wanted to melt away before my rod but then again you may be using older/thicker sheet metal than I was.
Jason
Everlast 255EXT - Perfection
Everlast PowerPro 256 - UPS Demolished
Everlast MTS200s
12 Ton Shop Press
DeWalt Hand Tools/ChopSaw