I would think that if your chain is rubbing the track with the 12, the 13 would fit just fine. I'm running a 12 currently and it isn't yet rubbing, and a 13 easily fits, especially now that the chain has stretched a little.
What year bikes are you guys running? Maybe its the non linkage 300's that allow the 13 I'm in the same boat with the 13 not going on at all even with a used chain.