So I'm going off of the image provided in the Nemesis entry, and knowing that I'm going to be sinking a lot of trial and error into this, I've committed two imperial and two chaos cruiser sprues to this project. This is what I started with:

First observation: I lopped off the lower chaos cruiser hull way too close to the prow, which brought the second cruiser sprue into play within minutes of starting the project. I also went back over that imperial hull a few times to trim more detail off, as will become apparent in other pictures.

After sectioning up the first chaos hull, I trimmed everything flush on the underside; it's going to be floating over the engines and reactors section of the imperial cruiser later. What this picture doesn't yet show (you'll see later) is that I also opted to widen the two chaos engine vents to fit two single imperial engine nacelles where they were located (taken from the top two sections of the imperial engine cluster once I realized they were going to be buried under the chaos hull section).

The next quick realization was that, well, I needed to lengthen the dorsal chaos cruiser hull. My solution was to add a pair of supports along the inside to keep the hull's angles perfect. Then I completely destroyed the ventral hull of one chaos cruiser kit to get myself 4 more individual upper hull sections, two of which I then attached to my framework. I used a ruler to level them relative to the forward hull section, and made sure they lined up properly. Wasn't as hard as it sounds, and the plasticard strips gave me the perfect base to later attach a trapezoidal section of card to to brace and reinforce the whole frame.

This is what I ended up with, a partially-finished dorsal hull for the BB. I mounted the aft pair of gun decks so I could better true up the later sections (more on this in a bit).

Here's where I am now. I trimmed down the lower prow to mount a pair of chaos cruiser lance turrets under the prow armor to represent the 4 L/F/R lances. I usually take off the upper prow guns and mount a single lance turret beneath for my Murder & Hades cruisers, but for this I'm leaving the little guns on, in the style of most chaos GCs and BBs which feel like they have way too many guns modeled on for their actual firepower output

I brought the prow down to the ventral gun decks by using the imperial ventral fin. It's wider than you think, and plugs in just fine there actually. I'll need to bulk it just slightly with plasticard though, as the gun decks end a little too abruptly for my taste and need to taper slightly more on the prow. I also need to design four spacers to fit between the dorsal gun batteries ([stern][guns][spacer][guns][spacer][prow]), and I plan on decorating at least one pair with chaos symbols or legion markings. I need to build a lot more detail onto the dorsal spine, although I have the primary and secondary bridges ready to go, and I need to build up the ventral fin to match the book image too. I'm not going to bother trying to lop more off of a second imperial cruiser kit to put on there; I have enough styrene in various sizes and shapes from Plastruct that I shouldn't have any trouble making it nice and gribbly. I also want to try my hand at adding more engine nacelles onto the stern. 7 vents don't really feel like enough for a BB that can move 30cm in a turn under cruising power. I also have some gap-filling to do, and some extra detail to add, but I'm going to save the greenstuffing work for tomorrow probably, it's late enough and I still have to clean up around here.
My big design debate right now centers on how I'm going to connect the dorsal hull to the stern around the bridge area. I made a point of keeping the chaos cruiser sections level with each other across the hull, rather than dropping the rear section a little lower like the Nemesis image appears to show it. This'll allow me to wrap the upper deck all the way around to the stern if I want to. The original image just shows armor plating, but I'm not sure I want to go that route. I'm considering adding a pair of chaos hangar bay sections for a few reasons. First of all, they'll give me visual launching decks for planetary assaults (the rules don't have hangars, but I feel like if this is going to be a chaos marine ship like I want it to be, it needs to have embarkation decks. Just figure they have so few stormbirds and thunderhawks left that they don't dare risk them in void war and save them only for planetary assault). Also, they'll thicken out the ship around the bridge, and without them it's beginning to feel a little wasp-waisted, which doesn't look right. Along with that, I have a problem in that my dorsal gun decks are now much wider than the section of the chaos cruiser they need to meet flush with, and test-fitting chaos hangars has demonstrated a good trompe l'oeil effect, helping me actually narrow the dorsal hull back down enough to meet the stern safely without appearing to narrow it.
As a final pic, here's the hull so far alongside a Retribution-class BB for size comparison. I think my design is big enough to feel like a BB, but sleek enough to believably be an 8-hit BB with a propensity to erupt in flaming self-destruction.

P.S. This is my first post, and my first time really messing with Photobucket for that matter. I realize my pics might be massively oversized, and there's a ton of them since I made a lot of progress today. If I need to shrink things down or swap out my links, please let me know and I'll get on that immediately.