It looks good, but I would be worried about that critical point where the fork is attached. I've seen that design before, and always thought why couldn't that fork extend rearward past the frame, maybe 4 or 6 inches, that way you could gusset it there, there would be a bigger attachment area as well as greater leverage.
Just a thought, the light duty pallet movers TSC sells are just square butt welded (just angle iron) so I guess it must be pretty strong. plus you used a heavy wall box material.
Good idea, For the lower cross just use a horizontal draw bar, you would be half done right there, since it would secure the forks and already has the pins on the ends, Plus it's plenty strong enough.
JB