If you are not a very very good welder dont attempt to weld it yourself. Unless you are a good fabricator don't attempt to fabricate. The problem with tacking everything up yourself is that no two welders are the same. It may be clean enough for one or the gaps may be two large for the other. One important thing about mig welding, IT HAS POOR PENETRATING ABILITY. you really need to have a small gap between the two pieces you are welding together. With a real tight joint, unless your heat is really up there you won't have the penetration required for a good sound weld. Most mig welders don't weld anything much thicker than 1/8" thick. Been welding longer than I cared to. Leave it to the pros.