I am currently building my truck and I knew I would run into this problem so I have done two things. Installed a 4.88 ring and pinion gear set and installed a ranger overdrive in front of my sm-420 transmission. This combination lets me do 65 mph while turning less than 3000 rpm.

The gear sets can be found for about $450, here http://wellertruck.com/lit/Weller_Diff_Catalog_2011.pdf you can find the part numbers for the gear sets. When you find your part number go here and search http://www.truckpartsinventory.com

The overdrive unit was $1400 here http://www.advanceadapters.com/categories/complete-units/91/