One thing I like about building a true custom (not just bolting on shiny wheels and putting stickers on the windows) is that there are no rules. It's all about what looks good and what fits your vision of the final product that is YOUR project. One theme that I have with Project Pile House is to make the body less "busy" and give it a smoother overall appearance. These trucks were meant to be utility vehicles, so there wasn't much thought put into styling. Definitely not like their passenger car counterparts. All that anyone really cared about was that it was reliable, could haul a lot in the bed, and that the hood, doors, and tailgate closed and latched. So this means I need to fill and smooth a lot of body seams or body lines that are all over the cab and front end.
These seams need to be filled with metal, and should not be filled with body filler, no matter how tempting it is to just run a bead of filler along them. Occasionally you can get away with filling a seam by slowly stitch welding it shut, but this could require a few passes to completely fill the seam and it puts unnecessary heat into the panels around it. I've found that these seams can be easily filled by using TIG filler rod and a MIG welder. This tech tip should help you fill body seams quickly.
You want to start by removing any paint or rust around the seam, and then run a wire wheel in the groove to remove anything tucked into tight crevices. I found an angle grinder with a flap disc takes care of most of the process, but a thin wire wheel cleans out any remaining debris. If you're the overly cautious type you can spray some Self Etching Weld Thru Primer in the seam to help seal the area.
After you're down to clean metal, you'll want to find a TIG filler rod that will fill the seam and sit flush, or just below, the surface. You then want to set your MIG welder to a higher voltage or heat setting than normal for the metal you're welding. The idea is to produce a quick, hot spot weld that melts the filler rod into the seam and leaves a fairly flat weld on top of the panel. The flatter the final weld is, the less grinding will be required.
After you have a few spot welds holding the filler rod in place, you can then stitch weld the rod into the seam. Always remember to alternate your spot welds and allow the panel too cool in between welds. The seam should look something like below after it's completely welded.
With the seam filled, you can take a flap disc or low grit sanding disc and knock the "proud" welds down until they blend into the surrounding metal. You should be left with a seam that's filled with metal (and not filler!) and will require little bodywork when it comes time for paint.