I just completed an upgrade to a ff D70 on my 2012 E350 van converted to a motorhome. The conversion was accomplished with all new Ford parts, except for the differential gears, custom from West Coast differential, maintaining the 3.73 ratio. Load rating of the new axle is 7700lbs, and the wheel spacers are no longer needed. Upgraded tires to Continentals, rated at 3950lbs load. Email if you are interested in details,
gregswetzel@gmail.com.