If you don't like the medium cage, it will work fine with a short. You just have to run the chain slightly loose and not use the small/small and large/large combos (Which you shouldn't use anyway!)
You must be just around the corner from me (Stewarton). I have a similar themed Colnago ('94 Superissimo with Record Ti 9spd) and am building a Peugeot 753 Perthus for Pendle Witches / Eroica next year. Have been thinking about Wheelers, but with a gym membership, tri club membership etc, there's not a lot of spare cash left for more clubs!