You're way ahead of me. There's a tool you'll need to install the panels. I got mine at Harbor Freight ($30 on sale)on a recommendation of an instructor. Car Quest has a nice one for around $90 if you prefer that. It's an air powered flange tool, that forms a flange along the edge of one of the pieces so they overlap without sticking up on the outside. Cut the other to tuck into the flange. Use the other side of the tool to punch a 5 - 8mm hole about every 2 inches or so. Then take your mig and fill in the holes. Be gentle on the heat and don't go nuts. Now you have the panel in place and no major warpage. Grind gently on the weld to take down any lumps. Fill and smooth the joint, and you're ready to paint. I would hit it early with a good epoxy primer. Maybe others will tell you something different. MARTINSR is a real pro, follow his advice if he jumps in. (NOTE: I have heard that flanging on the thicker metal of these trucks is hard if not impossible. I can't say for sure, as I haven't really tried.)
What you've done is create a nice lap joint that is plenty strong and doesn't show on the outside.
This is not quite as easy as it sounds. practice on some junk metal first, then move into areas that you won't see if you are not perfect. Last, when you're real good, go for the exterior.
As you know from experience, less is better when it comes to filler. Buy a good product and use it sparingly. Good work needs little filler.
