FAT and C60X are listed in the "Vehicle Data Book" as having a total capacity of 42 gallons while most others are listed as 24 or 25 gallons. This isn't the same as the length ratio on the tanks you measured, so there must have been a cross section difference as well. They don't list a difference between 15 cwt, 30 cwt and short or long wb 60 cwt (except the C60X).
The AEDB design record gives most 15-60 cwt (including F60H) as 2x12.5 gal tanks, the FGT as 2x30 gal, the CGT as 2x20 gal , the F60T as 2x20.5 gal and the C60X as 2x20 gal. I suspect a typo on the FGT since it seems unlikely that Ford would make a 50% larger tank for their FAT when all other models have matching tank sizes.
I'm scared to look for another source unless I find a third, different answer.
It seems logical that the C60X with its larger engine and the FATs and F60T that would likely be more heavily loaded than the common trucks would be given larger fuel tanks to preserve a similar range between fills.
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