Maybe some info in here on aft axle tanks
https://www.fleet.ford.com/truckbbas...df/195-197.pdf
E-SERIES VAN AFT-OF-AXLE FUEL TANK
A Fuel System Modification Kit is available for some E-Series products for removing the midship fuel tank and adding an aft-of-axle fuel tank. This kit is distributed by:
Transfer Flow Inc.
1444 Fortress Street
Chino, California 95973
Phone: (800) 442-0056
Fax: (530) 892-0382
The installation of this kit is available as a “ship through” from
TDM
13000 Farmington Road
Livonia, Michigan 48150
Phone
800) 540-3913
NOTE: (1) The modifier is responsible for determining if the vehicle as modified with this kit meets applicable safety & emission regulations and is properlycertified.
(2) 2005 E-Series is Lev II Evap Emission. Additional information is available in the Ford Truck Shop Manual.
Also:
29.0 Gal. (109 L) With 141.8″ WB SuperCab/156.2″ WB Crew Cab
38.0 Gal. (143 L) With Regular Cab/158″ WB SuperCab/172.4″ WB Crew Cab
https://www.fleet.ford.com/truckbbas...essd-exlbb.pdf
I don't have a link to the source manual, but I don't see anything about an aft axle tank for the F250/350 in the pdf document. Still looking