is there just one length universal fit alpha one lower shift cable?

pretty much one length but there is old style and new style (determined how they screw into the helmet
) . I would buy one in person rather than online too so you could check to see if it slides smooth and easily.

Also, do not go aftermarket with this cable. I've had problems with the Sierra cables. They sometimes crimp the brass fitting too tight and they drag too much. Make sure it's a Merc factory part.