The difference between the auto. and manual trans speedo gears is the length of the gear. A manual trans. gear could be used in an auto., but not the other way around. It may not live as long because of the smaller contact area.
From what I can see, an AOD car has an 18-tooth gear with 3.27 ratio, and 17-tooth with 3.08s. So with 2.73, try the 16-tooth, which is Ford # D0AZ 17271 B.
cheers
Ed Nicholson
SCCoO