Well I went to Ventura on my trip down south and visited National Parts Depot. I picked up some items that I needed and I figured I could save some money on shipping. I bought some large items like preformed steel brake lines and a steel rear seat trunk barrier panel. These were bulky items, and anyway I was just curious about the warehouse. It was located in a light industrial area. Neat, clean with some items on display. There were several guys manning the phones to take orders and to handle walk in customers. I had phoned in my order but they held back on actually filling it until I showed up.
I bought that steel divider panel after reading online about the hidden danger of the gas tank in the early Mustang. The tank drops into the trunk and forms the floor of the trunk itself. There were some online articles and discussion of how in a severe rear end collision, the tank could rupture and send gallons of gas into the trunk then through the rear seat back into the passenger compartment. The trunk is only separated by the upholstered back cushion with a paper type seal. If ignited the fuel would cause a horrendous fire which has been documented a few times. This resulted in some deaths and serious injuries. When you consider how many million Mustangs have been sold and how few documented cases have occurred, it doesn't seem like a huge probability. If you remember the flap over the exploding gas tanks in the early Pinto, then you suspect that many cases were settled and kept out of the media. Ford states that the Mustang complied with all current standards at the time of manufacture. I don't doubt this, as there weren't a lot of things that were covered by safety standards in those days. Any one who is familiar with the Mustang community through experience, magazines, or on line, won't have heard a lot on this subject. I myself had not even heard or thought about it until a couple of months ago.
The practice of having the fuel tank form part of the trunk structure is somewhat unusual. Most vintage cars have the fuel tank attached by straps under the trunk area. The tank is separated by the steel trunk floor and it appears that if it ruptured, fuel would spill out under the car. A lot of modern cars locate the fuel tank ahead of the rear axle where it is less likely to sustain damage in a rear end collision. Very early cars had the fuel tank mounted under the driver's seat or in the cowl area below the windshield. This was done to assist in gravity feed to the carb.
There have been several methods of minimizing the potential hazard. There is a steel cover that bolts over the top of the gas tank which puts a layer of steel between the tank and the trunk compartment. Not a bad idea, but it is bolted on and I imagine that it could be bent and become ineffectual in a serious rear end collision. You could replace the tank with a fuel cell which contains a flexible bladder within. Many types of racing cars are required to use them. This is a pretty good idea, though they are expensive. I think that the steel seatback panel is a pretty good idea. It may prevent intrusion into the passenger compartment by fuel or heavy items in the event of a collision. If it is insulated it will probably reduce noise and it may even stiffen up the structure a bit. I have seen articles on Mustangs where the owner has fabricated a steel trunk floor cover of their own design. This couldn't hurt. I suppose someone could design a replacement system that would replace the truck floor and utilize a strap mounted fuel tank underneath. I will post some pictures when I add the steel seat back panel to my car.
Anyone who is familiar with early Mustangs knows that there just isn't that much metal in the rear structure. The rear bumper is pretty much just decorative. I noticed that when I placed the full size spare tire in position, it nestled up very close to the rear tail light panel and the rt. side rear wheel housing. It appears to me that the inflated spare tire was possibly placed in that position to reinforce the rear structure. The inflated tire would provide a lot of resistance to being crushed and would transmit the force into the wheel well area. This is just my intuitive feeling, I don't have any way of knowing what the designers intended. My conclusion is that it is a good idea to keep the full size spare right where it is at all times, It couldn't hurt.
On a final note. Nothing that has been discussed is to imply that the vintage Mustang or any other vintage car is a particularly dangerous or particularly safe vehicle. They were designed to the standards of the day. The earlier the car, the lower the standards for safety. There weren't that many legislated safety standards until the 70s. When we are driving a forty or fifty year old car we are operating in occupant safety conditions that existed at that time. Sorry to end on such a serious note. However there is lots we can do to improve our old cars and make them safer at the same time.