the right pitman arm

I have 71 Plymouth duster slant six car with power steering and i was looking for the right pit-man arm they sell two different kinds with different spine numbers ... so i am not sure which one to get i am guessing the difference is manual and power steering gearbox? or maybe the slant 6 got a different pitman arm than the V8 models ? any one know the right spine numbered pitman arm for a slant 6 car with power steering ?