There are many ways to achieve your goal.
1) 3.38 gears to put in your rearend
2) Change out the rearend for another with more gear choices
3) Install an OD transmission
I have a 59 gmc 1/2 ton longbed, and I would reccommend option (3)
Changing the gears also changes the ability to pull a load and the ability to accelerate. Put an OD transmission in and you can have your cake and eat it.
