Here's what I have in mind for the Impulse Engineering room. The guts of the Impulse Engines will likely take up most of the empty space, with the spherical reactor sitting on top of the vertical intermix...thing.
There's also a diagonal jeffries tube leading off from this room which follows the spine of the dorsal fin all the way down into the main engineering room. Or you could take the lift... depending on how dramatic the situation is :P
Externally, the spherical reactor pokes through the hull and forms the blue dome on top of the Impulse Engines:
I've also been touching up the external paintwork today, and I added some windows:
The windows are actually just lights at the moment due to their location being at floor level, but I have enough room to move the dorsal decks down a bit internally so they can line up properly and be usable glass windows. That will also allow me to add the saucer seperation hardware, which will mostly consist of TNT
I do seem to have run into a snag though... I can't seem to find a way to have the turbolift reach the lower deck on the rim of the saucer. I can easily have a corridore open into the lower section of the Impulse Engineering room... but that feels a bit of an unlikely arrangement.... hmm...