One could also postulate that the "warp core" of the TOS movie ships was located underfloor at the bottom of the vertical "plasma conduit" shaft seen in the first movie, was at most three decks high and possibly just two, and basically always sat in that TNG style facility. It might have looked a bit different in E-nil-refit than in E-A aesthetically, but functionally it could always have been hiding in the lower forward part of the secondary hull. After all, we never saw the forward bulkhead of the cargo hold, and can push that aft quite a bit to accommodate such a warp core facility.
The other option is to have the E-A core replace the E-nil vertical conduit wholesale, as shown - and placing it aft and the turboshaft forward is what works better for the E-nil as well!
Why no space for tankage? A sizeable part of the connecting neck is porthole-free and might be a tank, not much smaller in relative terms than the E-D one. And there's always room at the bottom...
Timo Saloniemi
The other option is to have the E-A core replace the E-nil vertical conduit wholesale, as shown - and placing it aft and the turboshaft forward is what works better for the E-nil as well!
Why no space for tankage? A sizeable part of the connecting neck is porthole-free and might be a tank, not much smaller in relative terms than the E-D one. And there's always room at the bottom...
Timo Saloniemi