Eastwood sells copper backing plates with the magnets already in them. A durston tool is also useful sometimes. Once you start welding sheet metal with a backer you won't do it any other way. Backers aren't just for filling holes, I use them on all sheet metal joints when possible. When you use a backer leave a small gap in your fitment, then you are basically filling the void. This minimizes the finished height and requires less dress work before prime. As i said before, weld on the back side when possible. 23 wire(I use it exclusively for body work) centralizes the heat better, but you have to increase your wire speed. Doing sheet metal isn't hard with a mig. It's just about patience and consistency. Work slow, let it cool, and make the same tack every time. 99% of it is patience in panel fitment. Rushed panel fitment will lead to lots of mud magic even if you are God's gift with a welder.