If I recall I didn't have to change the drive shaft length but did have to change the yoke to go from FMX to AOD. Read this entire thread and I give links about AODs.
http://www.network54.com/Forum/119417/thread/1240542812/Drove+an+%2789+Mustang+GT+today%211978 Ranchero GT 351W C6
1979 Ranchero GT 5.0 HO, AOD w/98 guts, 3.50 9" Traction-Lok
Visit the Small block forum
http://www.network54.com/Hide/Forum/119417
Salt is for steaks-- Not roads!
Alcohol is for drinking-- Not in my fuel!