Terjemahan:What he had written was true.
OK. "What he wrote was right". Saya tahu ini bukan tempat yang tepat untuk memasukkan kalimat itu, tapi kalimatnya tetap benar nevertheless.
What he wrote was true should be acceptable.
There are another acceptable answer with slightly different meaning in English.
What he had written IS true. It means that what he had written is still true now as in the past.