Like72fastback said, a wire feed welder will allow you to weld panels together, but the welds are either going to be seams or plugs. Seams are where you weld along the edge (and definitely would not look correct on a Mustang) and plug welds typically get ground down to disappear. The holes for a plug weld are either drilled out through the existing metal being kept or drilled or punched out of the new piece to be installed--this all depends on how the metal is situated.
For welders, MIG (Metal Inert Gas) works quite well on mild steel. A friend of mine used a wire welder with flux core and he got decent results, however I would prefer to have the gas. Many of my customers have spot welders that have two contacts that are pressed together to put a little bit of pressure on the weld area as well as the welding current. Those welders form a perfect looking factory type weld in most locations that does not need any grinding or finishing. For the home hobbiest, these welders are financially out of reach and a MIG is a more economical solution.
