Certainly Tony M's concerns are valid. But I know there are shops doing this.
My fleetside bed looked like someone had used the one side for cannonball practice. I decided this would be a good place to try this method. I bought the rear bedside panel (goes from end cap to center of wheel well, and from bottom to seam approx 8" from top of bed rail. Also bought the front that goes from center of wheel well all the way up to the back of the cab.
At the back of the cab I left about a 3" strip of the bedside up to the seam the same down the the length of the bed along that seam to the tailight end cap. I separated the the spot welds holding the end cap to the bed side since the panel I received included that flange. Basically, I cut the whole bedside out including the wheel well, leaving a 3" frame.
Using the duramix system, after trimming and prefitting the panels, I glued the the panels at the top, the whole length of the bed, butt welded the seam above the wheel well and spot welded the tailight end cap to the rear flange of the rear panel.
When I look along the backsides of the panel, the duramix had oozed out along the seam the length of the bed, so I don't expect moisture to reak havoc-I painted the back of the panels with por-15 for extra peace of mind.
Will I do it again? I think so. I had very nice Schott panels to work with that were very straight. If I had had to weld that seam the length of the bed, they would have no longer been straight, I can tell you that!
I can't tell you how it will hold up because the truck isn't finished and hasn't seen any "road flex"...