To my (limited) understanding, 365 GTB/4 might’ve been first production (12 cyl.) Ferrari to feature fiberglass “tub” (=firewall, floor, cabin rear wall, rear inner fenders + boot/trunk front wall). This “tub” is/was made of 2 pieces with seam running (approximately) between A-pillar bases. Like anything else automotive made of fiberglass, they’re not known to “become brittle or deteriorate” expediently. If anything and in general, most sheet metal or steel structures near or next to them tend to corrode/deteriorate at faster pace. Also worth keeping in mind that many or most U.S. delivery (& perhaps later non-U.S. ?) 365 GTB/4s were made with extra sheet metal trays supporting additional heat insulation (=asbestos sheeting) below the the fiberglass floor to alleviate excessive cabin/floor temperatures radiating from exhaust/mufflers. These insulation support trays were installed by welding them directly to the lower chassis/space frame structures prior to adding the insulation sheet, fiberglass “tub” installation and eventual body assembly. Looking up from under the (assembled) car these “trays” may give an appearance of steel floor mentioned in auction description.