Not having had one of those years...

There will be a switch of some sort mounted either on the steering column or ?? to activate the overdrive.

There needs to be power routed to the overdrive for the so-called 5th gear.

Path may include a relay that is controlled by the switch on the column.

Power path to a solenoid on the overdrive is through a switch mounted on the transmission that is closed only in 4th gear. It is not the reverse switch located elsewhere on the trans. I strongly suggest geting this switch new while the tranny is out. If this switch goes bad, you will not have overdrive without a risk of serious overdrive damage.

As you pull the trans and etc, trace back the wires to get as much of the original as possible.
Somone else who is a lot motre familiar with the auto will be able too tell you what and where the items you need are located.

Duane