Congratulations, that is an elegantly simple solution to a difficult task. Forgive me if I'm a little slow on the uptake, but I couldn't tell by the picture just how you attached the 2x4 at the wheel. Looks like another block of wood, maybe with a notch in it?
I'm also curious, as the others, as to why you need so much extra weight (400 lbs. each side) above and beyond the weight of the filled tires, and you wanted to add even another 200 lb. ring!