John,
Just caught up with your build. Very nice to. I have built a few of these kits over the years, the last one in S7 and the floor of the kit is not correct, as you say. I was lucky, as volunteer at Locomotion I was able to take some pictures of the preserved example and modify the last few. I mentioned this to Rob Pulham last year at one of our meetings and he posted some of his pictures on his Flickr site (see above - pages 6 & 7). Don't know where you live John, but if it is anywhere near Shildon we are having a NE Area S7 meeting at Locomotion on Sunday Jan 24th from about 10:30 and you would be more than welcome.
Peter