Working on my aftermarket Boxsides - need some ideas/input

[Mattchu60]

I will start by saying I am new to body work and learning as I go. I bought some aftermarket short boxsides for my 1987 pickup a few months ago. I finally got around to installing them this past week. I have them mounted up now and also installed the tail lights. The tail light gaps are pretty bad on the top, its not level across but lines up on the side and bottom - not sure how to proceed on that one - was told body filler to fill the gap is a bad idea as it could chip out.

Right now I am trying to focus on the actual sides of the panels - the driver side is fairly flat, however the passenger panel is very wavy. In the area above the gas door there is a dip of around 1/8-3/16" for around a 12x12 area.
This area is also easy to push in - there isn't much structural strength.

One thought I had was to drill a few holes in a line, and take like a 1" wide piece of 1/8" stock and plug weld through the drilled holes it to the backside of the panel in that spot, my thought was it would give that area some strength as well as help pull the low spot back flat.

Is the above a good/bad idea? Or should I just resort to using body filler through this whole area instead and nothing welded to the backside?