I thought I read about correcting the space in Vance Bass' review of the JS cars in the June issue of Garden Railways magazine. He also talked about repainting the interior and correcting the weak coupler post problem.
The short answer is there has not been one OTHER than Kevin Strong's (EBT) rebuilding of the floor, and with that a new piece under the door.
That topic is as mentioned above by Jim typ.
I suspect that it is a manufacturing mismatch; the body actually fits over the back of the floor between the end steps and drops over the floor and because the under door area is slightly less that the actual dimension there = a gap. It is quite a tight fit.
If you have the coach in its 'as bought' condition I would suggest that you try some pieces of scrap plastic card to find the nearest thickness; then (having taken the body off, add some cut to size pieces of that thickness, and then match the color for painting, any slight mismatch can be hidden with a small amount of weathering in that area and the floor between the end steps.
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.
One "fix" I had read (I think on the 1:20 yahoo group) was to cut away the red plastic right around the tabs for the sides, so the sides attach to the metal plate, not the plastic floor. This--in theory--raises the floor relative to the walls, supposedly eliminating the gap. I tried it on mine, and found it to be ineffective. I may have overlooked a step in the process, though, since I was doing this only as an exercise. I had pretty much decided upon opening the box and putting the car next to my other passenger cars that the end platforms had to go.