I think the move to stagger rear shocks came with the mid 60- early/mid 70s on the mustangs and then the Camaro /novas that were track raced with the boss302 and sbc 302 in that road racing era of the small block muscle cars---spread to the bigger engine mustangs and others---later Ford even went to 4 shocks on the rear of mustangs--don't know what year off top of my head but they had two shocks normal location and two more above axle length wise with otter frame rail---------