This was my take on the internal arrangement of the U.S.S. Enterprise that I did partly for my portfolio and partly as a mental exercise. It seems like a rite of passage that Trekkies interested in ship workings do a pass at the original, but I felt that others had pretty well nailed what the internals would be like better than I could, or at the very least I couldn't come up with definite solutions to inconsistencies as presented on the show and from various sources.
I ended up basing this take on the notion that the TMP Enteprise was 'a totally new Enterprise' as said in the first film, but rather was more or less arranged the same as its TOS counterpart. To that end, I retained positions for many ship's features as defined by Andrew Probert's work on the TMP ship - the cargo bay and the warp core most notably, as well as ventral and aft phasers.
For the TOS ship though, I imagined that it was more divided and sectioned up than the TMP ship, and that the TOS intermix chamber was basically hidden behind bulkheads, whereas for TMP the new one installed was opened up for easy view and access, with a new engineering built at the front 'junction' and the old engineering mostly becoming devoted to the impulse deflection crystal stuff.
I figured the two odd shaped hatches on the bottom were cargo hatches with narrow tunnels leading into the cargo bay, a feature eliminated by the connecting of the shuttlebay to the cargo bay in the refit. I also placed the oft-mentioned 'recorder marker' launch system in the small square hatch at the aft end.
I think that in this version, the TOS ship wouldn't have anywhere near the 'interconnectivity' that the refit or later ships would. And truly, the only similarities here would be arrangements and bulkheads. Depending how you want to believe it was rebuilt, you could argue that only key structural pieces were kept.
I am working on creating a new version of this TOS cros s section (as well as hopefully a full set of plan drawings) more in line with 'conventional' thinking of how the ship's innards are arranged as well, based particularly on the work of the ship's designer, the late, great Matt Jefferies, who actually put a lot more thought into the way the ship would work than most people assume.