There has been some debate on the gas tanks used in the Mustang II and as to what tank will fit what cars. Here's what my research has found and I've provided pictures and details to show how and why the tanks differ. The first design tanks were used from the beginning of production to sometime in  '76 but I do not know the exact date the changeover was made. The second design tank was used from '76 to the end of production. In the following article I show how to differentiate which tank you have.

If you are old enough to remember the exploding gas tanks on the Pintos, then you know the reason for improving the tank design. Since the Mustang II and the Pinto share the same design features of the rear chassis, the improvements to the Pinto fuel tank were also applied to the Mustang II. The most obvious change was to add a plastic shield to one of the tank straps to prevent the bolts on the differential rear cover (6 .75 rear, not applicable to the 8") from ripping the tank open on rear impact. Another change they made was to change up the very back crossmember of the unibody to be more angular, rather than squared off, to prevent the crossmember from creasing on impact and making a pointed edge that could also puncture the tank.

If you have a 74-75 model car, you have the early (first design) tank and if you have a 77 -78 car you have the later (second design) tank. If you have a '76 model the easiest way to tell which tank you have is to get under the car and look for the thick plastic shield secured by the drivers side tank strap. This will tell you that you have the later model tank. However, I've found sometimes the shield has been removed or broken off so it's not quite so easy but there's also another way to confirm which tank you have that I will describe later on. From sheer trail and error I've found that the early tank will NOT fit the later model chassis. However, the later tank will fit in the early chassis but it's not without problems either. Follow along as I point out the subtle differences.

Here's a view of the top of the tanks. The late model tank is on the left, early on the right. Not much difference seen here in the pics:

Here's a bottom view. In this pic the early tank is at the top, the later tank on the bottom. Again not alot of difference but there is if you look closely:

In this picture I show the other method for confirming for sure which tank your car may have from looking under the car. Notice the tank on the left has the sending unit inset into the tank. This is the early tank. The later tank has the sending unit flush with the outside contour of the tank.

Here's where we find out why the early tank won't fit in the later cars. In this pic the late model tank is on the right hand side. Notice one side of the top of the late model tank is more tapered. This is to make room for that more angular crossmember I mentioned earlier. This is the problem with getting the early tank in the late model car. I actually found a car in the junkyard that had the early tank in a late model car and to get it to fit they used a hammer to "modify" that part of the early tank so it would go up under the car. Not only did it look terrible but it also made it leak from creasing the metal.

Here's where we find the problem with putting the late model tank in the early cars. Notice the marks on the tank where the straps are. The early model tank is on the bottom. Notice the straps are about 1 1/2" further apart and not in the same spot in relation to the tank, they are both further to the outside edge on the early model tank. The bottom of the tank is also shaped differently, which is actually more apparent in the third picture, which means the tank straps are also different. While the later tank will fit under the early car there is some issues with the straps aligning properly.

They are now talking about releasing a reproduction gas tank but I don't know which one is being reproduced. If I was to guess, I would say the later, improved design. Just make sure when purchasing a new tank you get the one that fits your car. Hope this helps clear up some of the confusion about the gas tanks in the Mustang II.