The body shop that painted my truck used pop rivets to attach them to the bedsides, and aligned the rivets so they were inside the bed pockets and under the fenders. It would have worked out fine, but they placed the strips a bit too high and I had to remove the rivets. My intention was to re-rivet them once I had the bed in, but they fit so snugly against the sides that I left them without rivets and they're just fine - except for the holes from the rivets. It's not that noticeable.
Based on the above, I'd recommend bolting all the strips down through the bed, then check the sides to see if they stand out. Then you could pop rivet them or just leave them. I think spot welds would discolor the stainless.
You can see mine in the gallery.
