I have only a couple of suggestions. First, you might want to do something to add a hangar. Atra's D5 had it in the standard Klingon position. I moved it to the back end of the main body by altering the original textures to suggest a door. If you want to use the kitbashed intake/vent section instead of the original but with the original textures, then the ship has no hangar.
Second, I stretched out the flaring bits on the neck. The windows with the original textures will look off.
If you want to use this instead of Atra's original, you might want to at least modify the dorsal "bulge thing" on the pod to conform to the TMP style, and maybe alter the intake section. In my game, by the TMP era these ships would be too old to keep up, so new ships would be built to take their place. I don't see the new war cruisers being replicas of the older ships; even if they kept the same basic design they would have more modern parts.
That is a good job of matching the different textures. I especially like the nacelle textures. They look much better than the original. Are you going to release this?
How good are you with mapping textures? The wing textures are reused and mirrored, so I couldn't add decoration to the wings. Would you be able to remedy that?
I can't believe someone is so interested in my little kitbash.