Regarding the use of a Mig welder on the outer sheet metal, one would want to use .025" or .020" (smaller is best) wire with the Carbon Dioxide/Argon (75%/25%) mix. This will give best control and least "weld spatter" (those little metal beads stuck around the weld).
.030" Mig wire works best for chassis.
Mig has another advantage in that it is more tolerant of pollution in the weld area such as a (minimal !!) ammount of undercoating, grease/oil, paint and galvanizing.
Torch is indeed best on outer panels but takes a high degree of skill that many are not able to obtain in a timely fashion.
Tig works well but is brittle. Use coat hangers for welding rod because they are made of very soft steel.
Cheers, Paul V