Hook the speed sensors up with the correct wiring to the computer. This way, no special switch is needed to get in/out of overdrive, just pull back the shifter one notch, it's out of OD, push it forward one notch, it's back into OD.
I didn't do anything special to get the OD to work, just hooked up all the sensors and made sure the linkage lined up correctly. My shift set up in the car is P-R-N-OD-D-L.
1966 Mustang, Wimbleton White 5.0L EFI/AOD
Ancient Ford lawn tractor
Currently on my 5th Mustang
Go Navy! 1985-2005