Nice bike indeed. Since Shimano uses a larger barrel, you can use either shift cable (which is nice), and you'll notice right away if you accidentally try to use a brake cable.

FWIW, downtube shifters seem to be really easy on shift cables, so there isn't much urgency to it unless the cables are already fraying. I'm morbidly curious to see how long I can make the rusty ones on my '91 Bianchi last. They might be original, for all I know!