They had a Columbia 2 speed rear axle, as an option. Where you could find one today is another question. There is also the new Mitchell overdrive for $3500, special order, 16 weeks to delivery.
https://www.macsautoparts.com/mitchell-overdrive-kit-ford-32-65234-1.html
Anything else, like a 5 speed trans, is a big problem because of the enclosed torque tube drive.