65 wildcat

On the passenger side of the transmission, there should be a tag riveted to the case. It will have a 2 letter code.

 

Came behind the 401 with 3.07 rear gears. It should be a switch pitch.

 

http://www.buickperformanceclub.com/switchpitch.htm

 

Sometime in the last 56 years, someone converted it back to fixed pitch. Not unusual.

https://www.v8buick.com/index.php?threads/vp-converter-trans-visual-id-guide.143723/

 

If you still have the switch that connects to the carburetor, then it would have controlled the switch pitch and kick down. You'll need to find which contact on the switch goes 12 volts at wide open throttle.

https://www.v8buick.com/index.php?threads/65-cat-kickdown-switch-pitch-etc.358669/

 

That means the transmission is no longer a switch pitch. You can only hook up the kick down.