I got sidetracked over-thinking what the engine internals would look like between the engine room and the hangar complex. I had thought it would be clever to look to the refit-Enterprise and see if some of that internal structure could be leveraged...
So after trying in vain to match up the cargo deck scene in TMP I found on Andrew Probert's website a reference plate he used for the matte painting and was able to do a match on that. Whew!
What I'm currently thinking is that like the refit-Enterprise, the TOS Enterprise's M/AM reactor is fairly small and the energy is amplified as it makes its way through to the warp nacelles and back to the main energizer which is further amped by the dilithium crystals.
Anyway, will return back to the TOS engine room after this detour