I would think that the alternator would have been part of the engine assembly sent to Hurst. This is a 1968 picture of a SS Dart, note the dual grove pulley and the belt running on the outer grove. Also it looks to me like the grooves are deeper than normal.

I also believe that it is not the dual groove pulley used on 69-71 hemis as there is to little space between the pulley and alternator.

Did SS cars use the 46 amp alternator? Do to the large battery and transistorized ignition.