Re: Replacement/auxiliary fuel tank
Yup. As mentioned, you can use the rear 35 gallon tank by flipping a crossmember, and adding the support hardware. A 55 gallon will fit by moving the crossmember way back. The newest cutaways have a 40 gal rear tank. I don't know if it fits as well as the 35 gallon. Stick with the same year donor chassis, because Ford has periodically changed tank opening and fuel line fittings over the years. Beware of inner paint de-lamination problems on the metal tanks. Eventually Ford switched over to all Poly tanks, but I'm not sure when.
Beware of rear axle weight issues, especially with stock size tires. Even with upgraded tires I wouldn't do it on a semi-float axle, so do that upgrade first if it doesn't already have a full-float.
2000 E450 dually V10 wagon