You have left out alot of pertinent information- engine? 12V? 24V? 24V/Common Rail? 6.7CR? Trans? auto-manual/2wd-4wd. Fueling mods done? Usage of truck? Gearing?
It's easy to say that you need a 64 or 66 but you may get away with a 62. like stated you left out ALOT.