It's hard to tell because I have no idea what components were used to make up this shaft. When I was building drivelines, we always built them, so the installed length was at mid-slip. Let's say you had a 4-inch slip, we would have it set up 2 inches each side. That way the shaft had equal travel when pushing into or pulling out of the slip yoke.
With the driveline in right now I would measure from center of ujoint cap to center of ujoint cap. This would give you the installed length. And tell your driveline shop this is the installed length, and you want it at mid-slip.
