The dual range Hydramatic has a very complex balance between main line hydraulic pressure and throttle pressure, so tinkering with the RPM/road speed relationship just might create some unintended consequences with the shifting. Proceed with caution! A taller tire might be a better approach to getting a little more road speed without messing with the rear axle ratio.
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!