I've added that stern portside doorway to the upper level engine room and matched it to the screenshot. It's about 3' wide from my estimation.
I also rotated a copy of the upper level blue corridor around the vertical shaft so that it is on the stern (and offset to the port). On average, there is about 30' between the corridor to the outer hull (measured at the floor level, alot less by the ceiling.) I'd say you could have machinery and turbolifts running back there.
I've included a comparison of roominess between the 305m and 355m hulls.
Click to enlarge