Just out of curiosity, why did you mount the block part of the Nissan assembly inside the box instead of outside? It would seem like there would be more of a positive lock to the rail with the block tab sitting in the slot on the rail.
Can the sequence for attachment be:
1. Flat square piece that goes in utilitrac grove
2. Grey block that secures #1 to track
3. Tool box
4. Washer, lock washer and bolt
I realize this configuration would raise the tool box a tad, but it might be easier to slide and provide a more positive lock to the track. Or, I could be full of it and don't know what I'm talking about.
That's a nice way to go for storage. Is that a trailer tongue box?
edit: I looked at your install again. Does the weight of the box prevent it from sliding if you place the "top" block under the tool box? Is that why you mounted the "top" block inside the box?