That is a common area for cracking on Mustangs, but I'm not sure yet why they left it that way....and the factory welds were not good either.
Would it be better welded----not sure yet.
I am sure of one thing about the original quarters you have, they are going to rust around the wheel opening, if not already. If you look at the back side of the quarter panel, at the wheel house, it might give you an idea.
You could also check the wheel house where it meets the quarter and rocker panel.
You may have some rust showing on the outside around the wheel opening.
The problem is that Ford put foam in there, possibly to keep water out, but it just trapped water in.
The lower areas are more prone to rust, but the foam goes most of the way around. So for that reason, and this quarter had some damage close to the wheel opening, is why I cut that area out for replacement. The front and back of the quarter was in good shape and fit good, but the reproduction face quarter panel did not fit good in the front or the back, but fit this area very good, and I can get to the back side when cut like this.
The repo quarter fit this whole area very good, including the pinch weld area.
I found a pair of NOS quarter panels before welding this in, but that is another story.