Should I get the 3.73 Axle Ratio from the SVT Package with my 2012 Mustang or should I buy that separately?

Im looking at a 2012 Mustang Shelby GT500 Coupe and Im building my own and im looking at the rear axle options and in order to get the 3.73 axle ration id have to pay for the SVT Performance Package which seems a little over priced to me... Should I just go ahead and buy the SVT Package with my mustang or should I get the 3.73 separately?