If I remember correctly there was a slight change to the top of the tank to accommodate for something. I know the early tanks don't fit the later cars though, I tried since the tank in my 77 was full of pinholes on top and I had a 76 parts car that I tried to swap from. From there I think there are only 2 - 3 options. 1st one is probably the most expensive as you've pointed out - find a replacement. A quick Google search revealed one through PartsGeek.com
here. Another option is to swap in a fuel cell, but that may be just as expensive and most likely less practical, depending on what you could come up with. The third option is what I did way back when - the Fox cars use a tank with a similar shape but required modifications to the car as well as the tank. I have a thread in the stickies under the tech link:
http://www.stangnet.com/mustang-forums/threads/fox-fuel-tank-in-a-ii.820975/
I also have a short YouTube video that I tried to illustrate what all needs done.
View: https://youtu.be/pEHGqIu7FJQ?si=04XOC7mG8Nq1Ezqm
That path required welding on the fuel tank. Be careful if you try this on a used tank as I did, but I didn't have any problems. I got my tank from a salvage yard, but there are plenty of options for new. Mine was from a fuel injected car, but I kind of wonder if you were to get one made for an early non-EFI car, say a 79, if the filler tube would work and you wouldn't have to really do much to install the II's pickup. Those can be purchased new from LMR. They have 2 options for non-EFI, the early
79-81 and the
81-86. You could also opt to install the later tanks and just use a Fox pickup and pump in conjunction with a fuel pressure regulator, but IIRC the fuel level sender has different ohms settings as compared to the II so your gauge might not work properly.
I guess there's a 4th option that I know has been done, but that requires even more modifications to the car: cut the floor from the hatch/trunk and install an early Mustang tank. That requires relocating the filler, using a flip down license plate and sealing off the original filler.
You are correct, the 74s had the filler neck located lower on the quarter than the 75-78 cars, but to my knowledge the tanks are the same from 74-76.