The Japanese terms for AM/PM are
Some real clocks widely available in Japan use them:
But what kind of clock are you building? What do you mean by "clock all in Japanese" and what do you mean by "just translate the letters?" Perhaps you are thinking of building a real "kanji clock"?
Then go ahead and use 午前/午後. While it does not suit Japanese people's taste at all, it probably makes sense as a souvenir for foreigners. Before doing it you may want to read this article carefully. Reproducing a single Japanese character can be a challenging task for those who do not understand the language.
Japanese people generally do not like to wear something with kanji on it, and even domestic Japanese clock/watch makers like CASIO mainly manufacture English-only products. Even small Japanese kids understand
AM/PM, and you don't need to translate it, usually.