As Chris said, the size of the ring gear determines what "kind" of ford rear end (7.5", 8", 8.8", 9"). The nine inch rear has many (the most?) gear ratios available due to its popularity in racing (strong but heavy). The 8" rears have many ratios but the most popluar in street/strip classic mustangs are 3.55 (usually used with a C-4, 3 speed auto trans), 3.78 (OK for the street, manual trans), 4.11 (you better have an overdrive trans or don't plan on going on the highway).
take a look at the
currie web site for more details
-red