Looking at the front edge of the floor, do you think it would be beneficial to add a tab in the center to provide additional support to the floor? The angle braces may narrow the gap enough, just hard to tell.
I would probably add one along the sides between each floor joist too. Easy to do and could stop a potential squeak. (Maybe I'm even more of a over-killer than you?)
For the tie downs, seeing a pic of the material on the truck, gives me an idea. Why don't you just have a way to run a spring loaded bolt up through a couple of stake pockets? Would leave clean sides and would solve the truck anchor points.
