quarter pannel install questions

A butt weld if done properly is just as strong if not stronger than the surrounding metal. Some people use a flanger so where the panel overlaps the outer panel is not slightly higher that the panel it is covering. There are many ways to attach replacement panels and I would say it is mainly the preference of the person doing it more than one way is more right than the other. The main reason for using the body line is that the warpage will be easier to control than welding in a flat panel area. The thing I will add about overlapping is that moisture and dirt could collect between the panels and eventually help corrosion. If you do any metal work and you can't get to an area to apply a coating of some sort after welding use a weldable primer or coating before you weld.

From your picture it looks like he did a fine job.