mugsy is right,

Increasing top end cruising speed (a common upgrade) is a matter of changing rear end gears and/or switching out trannies for one with overdrive.

Many people here who want to drive their older vehicles daily (especially half ton, light duty trucks and sedans) want better cruising speed so this topic has been discussed in detail several times before. Do a forum "search" using key words like "overdrive", "top end", and so on. You should be able to pull up something that can help your case. If not just let me know.

-ftyler