I have one but it will probably be a bit before i can pull the wheels.
I agree a direct measurement will be best.

they way i did the calculation was to use the measurements rom the manual. If you take the difference between the center of the outer tires in the back and the mean of both tires int eh back- that should be the offset of a dually wheel. take that and add it to the center of the front tires and thats how much longer they make the axle so the same dual wheel will fit. that came to 66" I'm pretty confident it is correct because I found a measurement by soemone with a 1.5 ton and it was within 1/4" I think all that is different between the trucks front axle is something in the hub accounting for the small difference...

Hopefully I can take a true measurement in the next month or two. This really sucks not having the truck at my place where I can work on it even in any 15 or 30 min stretch I'm "free"

Tj