I think I probably used the longer cable for the mixture control, seeing as how the mixture arm is located farther forward on the engine. However, you will probably end up removing and reinstalling the cables multiple times in order to get the fit just right, so you'll have plenty of chances to confirm this.
Aircraft Spruce used to sell special AN481 clevis forks that were threaded for a 10-32 shaft, instead of the usual 1/4-28 cables they're usually sold for. That's what I used, however I don't see it in their catalog anymore. Maybe worth a call to their customer service line to confirm before ordering. (and let me know what you find in case I ever need to buy a replacement...)
Note: All the above is from memory, and therefore likely to be wrong!
mcb