You are correct that the shaft is fine, however the splines are a different count between the EMPI and OEM shaft. That means that whichever shaft is used/reused, you must also use that retainer. The only other parts are the outer cage and the balls. In my trial I could not get the cage and balls from one to transfer over to the other, but Vette had a different experience. That means that if you want to replace any of the assembly components, then you have to change the shaft. In either case, none of the components are the same. Different spline count, different ball size, and different mechanism to hold the retainer in the cage.