Speedo and cable problem

[Russ/40]

I'm chasing after a couple problems left over after completing my 29PU. I am using a repo Speedo cable and jacket. I have confirmed the compatibility of both ends of the cable as to the torque tube end and the oval speedometer end. The speedometer works when spun with a drill out of the truck, but not when installed. It acts like the cable too short. Is there an auxiliary end or something I can use to get this thing working? It is such a pain to get the panel back in, I really don't want to pull the dash again.