Fuel tank in my '69 (British) Ford Cortina WAS the floor of the trunk. It had a flange horizontally around it that fit into a big hole in the trunk and was held by several bolts. In case of rupture and fire I would have been protected by the fabric of the rear seat; there was no solid wall in the front of the trunk. I believe that the early Mustangs used a similar design.68RT said:A lot of cars including Chrysler (My 71 had the tank filler behind the license plate) had the filler located there. Very common in the past. Fuel tanks were almost universally under the floor of the trunk for years.
I read an exchange between someone thinking of restoring an early Mustang and a Ford exec maybe five or so years ago. The 60s Mustang fan went on and on, until the Ford exec suggested that he just buy a new Mustang: it would be cheaper, more efficient, and far safer.