Being a Civil Engineer and not a Mechanical Engineer, I can only comment on what I see. I assume others have the credentials to confirm or deny the stresses and strains involved. I repeat, the less welding on a frame, whether it is on a dump truck, backhoe, or trailer, the better it looks to me. The OP asked what advantage the stepped frame coupling has. I have not heard a definitive answer yet. If it is that the straight frame is deficient, then it would be an easy cure to upgrade the section modulus of the framing material.
