When I restored my '69 Z28 and replaced the speedometer cable, I had a conical tool made out of nylon rod material very similar in shape to the one pictured, and used bearing grease help allow the grommet to push over the tool and metal fitting on the cable. The use of an long open socket that is necked down may also work as DONCZ28 suggested. When I get home this evening after work, will look for picture to post of my tool. If I recall correctly, I have a sketch of my tool design I will also post.