Good job!
One thing I noticed is your dump truck style tailgate. I have used ones like that and ones that swing to the side. The ones that swing to the side can be more trouble to open up but you can haul big stuff like rocks, etc.. that won't fit under the tail gate you have.
Big tex has a neat setup that allows you to use the gate like a dump truck or swing open as you desire. This is the best of both worlds. All it takes to modify what you have is a long pin the goes down one side of the gate. It acts like a pin to hold the gate closed in "dump truck" mode, or a hinge for the opening to the side.
Maybe add a hydraulic cylinder to allow lifting the gate. This would help while spreading gravel.
Fred