Ideal Shift points for 904

Automatic upshifts are a highly personal affair.
But IMO, into third by 20, is too soon for all but grandpa-style driving.
Your choices for correction, if the linkages are right, are;
Less governor pressure /(a lighter fly-weight) or more throttle pressure/(KD adjustment) to suppress the shift.
Sooner or later, you will run out of KD adjustment and the carb will no longer go to WOT. At this point you will HAVE to go to the governor, or to reduce the Line-pressure ... which usually is not a good thing to do. In any case, adjusting the governor is easier.
You can tie back the TP lever at the trans, in such a way as to set the minimum upshift pressure, but you have to do it in such a way that still allows the linkage to push the lever all the way to the rear.

If you have a factory KD linkage, then you will see a long slot for the carbstud to rotate thru. This allows a lot of throttle opening before anything even happens at the trans. In this case your only option is to change the governor.