center link question

AFIK, the split is 73 -74 and 75 - 76. the 73-74 uses the same center as the 72 {basically}
the 75 = 76 uses the different center with the specific idler/pitman AND ball joint. I bolted a 74 disc setup on my 64 dart. the dart had the original center-pitman-idler. the ball joints I used were the 73-74 style. yes there are 2 spline sizes for the gearbox. on the 73-74 stlyle of center link. the idler/pitman mounts to the center from below just like the earlier drum cars. I really dont know the reasoning for the design change.