TCA did an outstanding job of explaining what I feel is the primary reason this guy's frame failed---improper COG. If the COG had been located directly over the rear axle, and not a number of feet behind, this particular failure would have never occurred---even overloaded as he was.
If the owner of this truck would have taken the time to visit a scale (unloaded and loaded), yes, he would have discovered his truck was overloaded. However, he would have also discovered his front-axle was being off-loaded hundreds and hundreds of pounds---indicating a COG far behind the rear axle.
IMO, it's not a simple case of calculating GVWR and payload and calling it safe. *Where* you place the payload (COG) is a critical consideration, too.