Yes the outboard pieces on both sides are not grooved where they go under the welded side strip. They can sometimes be a challenge to get under existing sides. Install them first, leave the bolted connections loose, if you need to you can carefully bend them up slightly. If you notice how these beds are built it is all about movement, between the boards, the eccentric washers with oversize hole for the attachment bolt to go through the board and the strip design. They grow, shrink and constantly flex, both loaded and unloaded. I have attached a folder of pictures that may help.
https://www.flickr.com/photos/advance-design_parts_co/albums/72157631884205225/with/8130479153/