A '66 was originally 19-spline but it's quite common to find them updated to the 24-spline input....the input shaft and the reaction shaft support must be swapped as a pair.
From a replacement parts standpoint, the main issue would be if the swapped parts were from a '67-70 or '71-later. '71-later had different sealing rings, piston seals and front drum bushing.
Thanks for the info. I believe the support 71 up has wider bushing, any way to tell the difference on the shafts and pumps? casting #'s on the pump or stator support? I think I am going to pull it apart tonight so I can get a better idea.
