No the 6x6 had a 13" clutch ,4x4 was 12",spline size and throwout the same .........the clutch was the same as used in conventional IHs of the period.......and as the flywheel was interchangeable between trucks ,some were drilled for both 12 and 13 pressure plates.......i would also add ,there is no reason a 12" clutch wouldnt be quite satisfactory in a 6x6.,or a 13" in a 4x4......Repco made all the 13" clutches ,IIRC,but they are interchangeable with standard clutches the same size and type.