It doesn't need to be shortened, the legs simply need to be moved to a location that will fit perpendicular inside of the stake holes. A foot ot two of overhang will not hurt anything if the legs are supporting the frame and not just dangling in the air.
Yes, I said dangling.
Now, as for this... I have no fWONking idea what the fWONk this is.
I think it's the front view. The green thing is the truck bed, and the shit brown part is the new leg and knee brace. The black thing is the existing leg?
All jokes aside, Dave is on the right track. Move the leg over to where it needs to go into the stake pocket. Leave the top long, and knee brace. This would let you load it almost to designed capacity, and you still get the overall length he needs.
Dave, we're giving you a hard time here, but you really can do this right without much work.