Mike, if you are going to use a gun, I would go with a DuPont Imron(sp?).
Sand and prep, use primer, and both basecoat and clearcoat. That paint is some of the toughest automotive grade paint I've ever seen. Doubt if you will get a good match on color there either though.
I wouldn't worry about it too much, as just about anything will get scraped off in a good crash.
PJ1 is the way to go if you want to both match color, and have a paint that is mfg grade. If you are wanting to just spot paint, then their spray bombs are the way to go. But again...I would go with any base primer, then the basecoat (color), and clearcoat that PJ1 offers.
The metal that got welded is going to be far more suseptable to rust than metal that hasn't been heated, which makes the primer step very important. You may think about finding a zinc primer.