Pressedun, Could you possibly build out the section where the tarp hangs over your tailgate? It looks like you could replace your vertical 2x4's at the end, add another piece of plywood to box it out and maybe your exit chute for the loader could be run through it.
I don't remember who, but another member has a setup similar to yours. He has a solid piece of wood to cover the rear and it has a hinge so it swings open when he dumps.
Good luck & nice setup!