So, with the fuselage closed it's on to the next step. This involves some surgical work.
In order to show what needs to be done here's a bit of a situation report of what it looks like right now. The first 2 pics of the big one-piece clearpart on top show that the fuselage halves mate up to the appropriate width. Fit the of clear part is excellent. In other words, this is the width i have to work with. There's no pressing inward of the sides to make the insert part fit, by doing so the fit issues would increase dramatically because the clearparts wouldn't fit anymore, which is far more difficult to fix.
The 3rd pic shows the upper deck insert part. As you can see it needs to be widened significantly in order to get a good fit between the two fuselage halves. The 4th pic shows the the upper-deck insert part from the side. It's pretty apparent it will need significant thinning as well to end up on the same height as the rest of the fuselage.
The idea is to first thin the part down to the appropriate thickness...by bringing out the 'big guns' (5th pic) so to speak, and then glue some plastic strips in place to widen the part and then trim these down so they'll blend in with the rest of the part. Hopefully this will lead to a good fit of the upper-deck insert part.
In itself the operation is pretty straight forward, but the delicate nature of the insert part makes working careful and slow mandatory. No hardcore grinding down of the part that is. But slowly take some off and test-fit, etc. etc. until it fits.