When I got my truck (also a 54), the side pieces were already gone, probably for the same reason you are asking about them. The bed side was in great shape except for the area where the bed side strips had been. about 75% of the side had rusted through almost the entire length of the side strip. I took my bed sides to a fabricator and had him cut the bottom of the sides off right above the rusted out line and weld in a newly fabricated bottom. They did an okay job. I had to fill in a little along the weld line where it was not flush. I am trying to decide how to attach the new stainless steel angel strips. If I spot weld them they will discolor at the weld. I am considered drilling and attaching them with Button Socket Head bolts. This would not be an issue except between the back of the fender and the rear of the bed. The nut side of the bolt would be exposed on the outside of the bed. Most of the nuts would be concealed in the fender well for the majority of them. I have put off doing anything because I can't decide on my best option. I will be curious to see how you solve this.

Dave from Northern Kentucky
My 54 3100