I've never given this much thought, I've never had to replace a cable or just the inner. I have disconnected the cable at the speedo end and noted that the collar, usually brass is at the speedo end. This locates against the shoulder of the sleeve and when tightened up also rotates against the speedo boss. This means the inner is positively positioned and doesn't have any in/out movement in the speedo unit. Does mean that you replace the inner from the speedo end. Don't over lube the inner. A rubdown with graphite grease and then another wipe to remove virtually all of it is good enough. Oil can end up in the speedo which isn't good. Outers normally have a short length of spring at the gearbox end to protect the cover from burning on the exhaust pipe. They can run very close together.
This link references a previous thread of yours but it does mention speedo cables. If you've not seen it you may find it useful.
http://www.sure-vent.com/M40Trans.htm