A few photos of the Transfer Flow behind/next to an NP272 transfer case. It's a tight fit; I actually had to cut the exposed threads off the front tank strap bolt since it was hitting the output shaft flange on the transfer case. Despite the close clearance, I've had this tank in for the last 100,000 miles and never had any interference with the transfer case.
The back end is a different story. It fits around my Sterling 10.5 rear axle, but when installing Agile Offroad's RIP, I had to relocate the upper damper mount on the cross-member about 1" to the left/back. Bilstein builds their dampers with the body on the bottom, while Fox puts the body on top. This difference necessitated additional clearance.
If you manage to find one of these tanks, a downside that you should be aware of is the weight. Not only are you carrying more fuel on board, but the tank itself is weighs about 150# versus maybe 30# for a stock stamped-steel unit.
2003 E-350 EB