You must buy the LSD unit (Tru-Trac is the Ford version of "posi"), then a 3.50 ring and pinion set and have them installed. The tolerances are very tight and should not be done on a workbench by a novice.
Ford basically sold two different 9-inch housings from the mid '60s onward. The small bearing and the large. Mainly F-series vehicles and Broncos got the large and passenger cars got the small. Measure the outer flanges of the two axle tubes, if they are the same you're all set.