Dave Therhino,
If they are essentially making a “wet fuselage section” aft of the gear well then resistance to external fire is only one of the issues. The design must prevent rupture of the tank in a wheels up, on-airport landing. The design of the landing gear must prevent penetration of the tank in the event of a gear failure, including due to an overrun into obstacles.
If memory serves, Airbus wanted this setup for the A340-500. The regulators wouldn’t buy it and the Airbus revised design incorporated a Kevlar bladder for this aft lower fuselage fuel tank.