Hi Andy. When you add the tracks, the hull top will not sit properly on the bottom hull, as there is not enough of a gap to fit the tracks as well below the fender. You will need to add a plastic strip over the whole lip of the the bottom hull.
Dry fit some track on the top now before you add the tracks, see what height you need to build it up. Build it up before you add tracks so you can tidy it all up!
This probelm is in all dragons JS-IIs. Hopefully you get this message in time!