The 18 spline unit came out for the Hemi and 440. It has a larger diameter input shaft then the 23 spline units.

The 23 splines on the 1" diameter input shaft look to have awful small teeth, where the 18 splines on the 1.25" diameter shaft look massive. (Shaft diameters are from memory and may be larger or smaller then stated, someone will correct the sizes.)

This would lead one to believe there may have been an issue on the shaft and spline size with the 440 and Hemi motored cars with good traction. Chrysler at the time overbuilt a lot of stuff. Gene