As I now routinely need access to the inside I have built a set of steps and filled one of the engine wells to create an easy route.
The floor beams are made in a similar way to the hull bulkheads. They are cut out to receive the bridge deck beams. The beams are dry fitted and lines are used to align the beams and to check the levels.
The top of the beams are notched for the timber to support the cabin floor. The beams are them fixed in position.