I' m willing to guess that 90 percent of the dually or C&C were 4.10 ratios. 3.73 seems more common in the Single wheel trucks. There is also a 3.42 ratio that seems to be more common in the late model trucks. There is also a 3.21 gear set out there, but you'll only find it used.
Be aware of the 4.56 rearends though, the internals are an entirely different animal.
You can use dually hubs on the 67" axles to make them 63". You can also use the Easton hubs with different bearings to make a 64.5" axle. Either way, you have to get creative with the brakes. Luckily, the van rearends have drums that are more shallow.
