So, more rework.
Started cockpit framing to check positions of fuel tank, etc, and found the Radio Op's frame is too far back on the Hasegawa kit.
Removed the seat (with added clip), the rear wall and shelving, and cut off the rear portion of the cockpit floor.
Pre-cut:
Shortened floor with rear wall added, and ribbing in fuselage (in progess):
I have indeed my friend! It's not millimetre precise, but there were angled plates at the edges of the cockpit floor which (when attached) will cover the bottoms of the ribs.
(And thanks, but you would have done this much nicer!)