I have a suggestion to help with this part. One of the modifications I made was to add some 2x4 bricks and plates under the 4x4 DBG plate you add on page 436. This keeps the top of the ship in place while you flip it over (and you can leave them afterwards and they add nice structure). Immediately after this, pages 437-446 build a sloped dark grey section. If you remove this section and have held the rest of the top in place with the bricks, you can easily flip the ship upside down. I then placed a box in the gap left by the section I removed, and a box under the connecting spine, and had no problem working on it upside down.
Hope that helps!