If you don't mind learning to use two shifters, there's always the option of installing an auxiliary transmission ("Brownie") in the driveline. Most of them are 3 speeds- - -under/direct/overdrive which turns your 4 speed into a 12 speed and gives you a higher gear for highway speeds when running without a load.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!