The "exploding" gas tanks were an issue with gas tank venting. Not sure the tank armor is going to help that. There are numerous articles about how to properly vent the fuel tank on vintage mustangs.
Installing a plate between the rear seat and trunk compartment would be much more effective. The Pinto gas tank crisis proved that.
The "fires" were only really seen on a very limited number of rare cars....the Shelby GT 500. The Shelby's in 1969 and 1970 had a center exhaust outlet in the rear valence, which was directly below the fuel tank filler cap.
From an early Ford article:
Under certain running conditions, when the carburetor was set just right and there was a rapid backing off of the throttle, the engine could backfire. It was known as “pop back” and it was unique to the GT500’s 428 powerplants. On a couple of occasions, this pop back ignited the fuel vapor cloud and actually set the tail end of the car on fire! It was incredibly ironic that one advertising tag line for the 1969 Shelby was “fire, and refinement” but that was not the kind of fire either Ford or Shelby Automotive had in mind. The fix was to install nonvented fuel caps and replumb the fuel tanks so that they were vented via a hose that exited at the bottom of the rear quarter panel, away from the exhaust exit.