Change the length of the throttle linkage from the carburetor to the transmission. I don't recall which way to move it, but lengthening the linkage will move the shift point up or down, and shortening it will have the opposite effect. DO NOT GO FOR AN EXTREMELY SOFT SHIFT! It will cause the clutches to slip and burn. A powerglide has two shift speeds- - - -too low and too high!
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!