There is a good video series from "Old Man Fixing Stuff" on YouTube on him building a '55.2. He assembles the bed floor and bed by himself. Bed sides and front panel start towards the end of the first video link below. The second one finishes it up. I can't recall if he uses fender welting or not.
https://www.youtube.com/watch?v=BMsfwybzVh8
https://www.youtube.com/watch?v=Af--nubnEz0