What makes you think it shifts too soon?
What axle ratio do you have?
What engine speed does it shift at when at wide open throttle.

Try a very light throttle position and see when it shifts 1-2 and 2-3.
Then do the same for wide open throttle.

Report back on your findings.

As for throttle linkage adjustment there is a correct way and a wrong way.
You never should have the lever half the way back at closed throttle.

And a lot of people don’t realize the external linkage adjustment also depends on a correct throttle lever adjustment on the valve body.