I've never measured them like that, so I can't say for sure.  The MOOG K8158 is the correct idler arm for your car.  The pitman arm seems to be for a '69 or later car though.  A '68 part would start with a C8 prefix, I do believe that the pitman arms are the same late '67 to '70 though (all 1-1/8" pitman shafts)

They could be different lengths, because each arm moves in an arc.  Both arms are also not necessarily mounted centered on the same line perpendicular to the centerline of the chassis.  If the pitman arm mounts farther forward then it would be shorter.  They key is that they move in the same arc; length is unimportant.  If their arcs were off it would pull the centerlink unevenly and the car could turn faster one direction than the other.  All of this was designed into the car, so I would hazard a guess that if you have stock and/or stock replacement parts that are correct for the year and application then they are correct regardless of any length difference.