Body panel replacement tips/advice needed

I'm no professional welder by no stretch, as a matter of fact I got some valuable info from a few folks who weld for a living. I have never had any luck MIG welding sheetmetal with flux core wire even when I have my welder turned all the way down. It makes a pretty hole though! I use 90/10 argon-CO2 mix when doing sheetmetal as Argon rich blend don't penetrate as much. On "regular" stuff I use 75/25 or 80/20 for more penetration and higher heat. If you are gonna do some stainless steel then you are going to definitely need a machine that uses gas (tri-mix) and kicks out some juice.