The question comes down to what rear end do you have. My 63 C20 has an Eaton rearend. The Eatons third member (pumpkin)drops out. If that's what you have the best gears your going to find are 3.93, very scarce. There are some 4.11 around. If you have the Dana you have a few more choices in gears. Though you'd probably find it cheaper to replace the complete rear axle with one from another C20 up to 72. This will do for a start I'm sure there are others that can add more info.
Larry
