Also, since your U-Bolts are long enough (and can be cut down if needed), you could simply buy the blocks from yoru local ford dealership or this site
2011 Ford F-250 Super Duty Leaf Spring Axle U-Bolt Spacer | FordParts.com
They should only cost about $45ea (perhaps less).
Be sure you ask for the blocks off of a 2011-2014 F250 that is the same cab/bed configuration as your truck.
The reason to stay in that year range is because Ford started to offer 4" blocks as an option for 15/16 models because some customers had complained about the rear of the truck slouching when loaded.
Corpus Christi, Texas
TT: 2016 40' Chaparral 360IBL at 14000lbs
TV: 2016 F450 Platinum 6.7 CC LWB FX4, B&W Companion Hitch