There are some "tabs" on the walls, towards the end of the car, into which the floor screws. The problem is that there is a sub-floor, (onto which the seats are mounted), that interferes with metal floor and the tabs. Once the car is dis-assembled, it's a simple matter to just trim the plastic sub-floor so that there is no interference between the metal floor and the tabs on the walls. You just need to do this on the ends, there is no interference with the other tabs along the sides of the walls. Then once the car is re-assembled you may need to put a thin shim into the groove in which the floor rides to push the floor up to close the gap between the floor and the bottom of the door.
It's not as hard as it sounds. It's pretty obvious once you have the floor off. It certainly looks better to have that gap gone.
Good luck,
John