Not sure if I missed this in the post, but nice job on welding up the frame for the truckbox, looks beefy to support the weight and looks like it's securely mounted to the square bumper. However the square bumper to trailer frame is the weakest connection point in that whole setup (unless you reinforced the bumper to frame connection and I missed it).
My fear would be driving down the road and that whole assembly would crack the welds and it would come flying off..still attached to the bumper!
I'd also be nervous about road clearance with the spare tire below.
Besides that, it looks nice.