1990 F250s didn't come with a shortbed. Reg cab long bed (133" wheelbase) or Supercab longbed (155" wheelbase) were the only options. The 16.5 gallon tank was only on the shortbed F150s in the front. F350 chassis-cabs and F-Superduties came with 19 gallon tanks front and back unless they had a diesel, then they had a 20 gallon front tank (except the DRW chassis cab F350 diesel, which had 20s front and back).

I've got the 1990 manual sitting right here...any more questions?

Jason