Not sure what the issues you are finding are, but I can tell you that when it comes to doing the floorpans, you won't be welding a continuous bead. Body metal is pretty thin, and the constant heat buildup will start to cause blowouts. Practice getting a good puddle in short (~1 second) bursts. Go slow, and keep it cool. There are some very talented metalworkers here that can give way better info than I but I will be happy to share what I can...