[RallyRacer]

It will take more that a repair manual and $20 to make a trans control module. It is more complicated than "wiring buttons"; talk to Lentech, they made a controller for drag purposes, but it was very limited and not a true manual shift solution.
You would need to build a programmable solenoid controller for what you want to do. That is way over your head and very impractical. There are things to consider like solenoid timing, shift pressure for the desired load and throttle position, etc.
The practical solution would be to get the SCT PRP or another programing solution that allows you to modify every transaxle parameter. Then you can plot out your up and down shifts exactly how you want them.