What extra stresses? They can't be much. Some incorrectly perceive a lever is at work, but if the extension connection is solid / strong enough there is no way it can rotate the Pinbox (it becomes one solid "cantilever"). The approved for Lippert frames Goosebox demonstrates this as it totally replaces the Pinbox.
I replaced my 25K lbs rated adapter with the Goosebox only because it was strong enough to "rotate" (Bend slightly upward) the rear of the kingpin plate (the flat piece of steel the kingpin is welded to). This only occurred on the VERY roughest section of post Katrina beat up I-10 in New Orleans I ever encountered a few years ago. This bend was so slight I lived/traveled with it for two more years until this past summer when another encounter with horrible concrete in IL bent it just slightly more so I became afraid the plate could someday fail or even worse, separate from the Pinbox, however unlikely that probably was. This was a real world "Physics" test that demonstrated the fallacy of the theory that the whole pinbox would rotate from lever action and damage the frame. The adapter was much stronger than the Lippert Pinbox Plate, which dissuaded me from just replacing the Lippert Pinbox.