I've heard that the fuel tanks on the post-N Fords varied in size and capacity but I never was sure exactly what the differences were. Now I have 4 different tanks here that I can compare. I'm not positive I know exactly which tank came out of which tractor (I got all the tanks at an estate auction last month) so if I say something here that's not correct, just let me know.
Here are the four tanks. From left to right, I believe we are looking at tanks from NAA, 800, 601 and 801. From what I understand the 600 tank would be the same size as the NAA tank, but with the bottom outlet hole in the center instead of offset like the NAA.
Here is a height comparison between the NAA tank and the 800 tank. The 800 tank holds about 3 to 4 gallons more than the NAA or 600 tank and is about 2" taller. Otherwise they are about the same shape and length.
Looking at the same two tanks from the bottom you can see that the NAA tank oulet hole is offset to the side while the 800 tank's outlet hole is centered. Also the 800 tank has several more stiffening ribs.
This shows the 800 tank (on the left) next to the 601 tank. Again, the 800 tank is taller, but the 601 tank is longer. I'm not sure of the rated capacity of the 601 tank. Maybe someone with quick access to an owners manual can post that stat. The length of the X01 tanks is what requires the different hood hinge and latch location differences between the sheetmetal on the two tractors.
From this angle you can see the extra length of the 601 and 801 tanks as compared to the NAA and 800 tanks.
Here is the 601 tank next to the 801 tank. Notice the extra height of the 801 tank on the right. It appears that it is designed to overlap the top of the radiator a little.
From another angle you can see the difference in tank height. The 801 tank has much more area below the side seam than the 601 tank. To partially compensate, the 601 tank mounting rails are taller than the 801 tank mounts.
Of course another major difference between X01 tanks and X00/NAA tanks is the presence of a mounting hole for a fuel gauge sending unit.