Sounds like adjusting may fix your problem because when you lift a little it also changes the position of the detent cable. The reason I said it might fix it is because there is also the vac modulator to consider,because the vacuum can come up enough to cause eariler shifts also.
To adjust the detent cable (passing gear),remove the air filter assy. There will be two cables hooked to the carb,drivers side. The top cable is the throttle, and the bottom cable is the detent. Some cables have a metal D shaped tab as part of the cable just behind the bracket that holds it in place,this is the later style which is self adjusting.This style cable press the D tab and pull the cable away from the carb, or toward the rear of the engine.The first time you give it full throttle it will reset to full detent.
If you have the older style cable there is a T looking part that needs to be pried up with a screw driver.Before you pry the lock up mark the cable so you will have reference to orignal setting.Move the cable out or toward where it hooks to the carb 1/4 inch or so until it shifts where you are happy this may take two or three 1/4 inch moves to get it right for eariler shifts.
If this don't work for you let me know there are other things to do also,but try this first.
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