Also, according to what alot of modern scholars say.....not that that in and of itself means anything, but they are saying, and I believe them.......that alot of Jews in Palestine around that time spoke greek, and not just Hebrew and Aramaic. Infact, some say that most Jews didn't even know the Hebrew language around that time.
A number of Jews in the New Testament actually have greek names (Nicodemus), and some of the landmarks were Hellenistic as well.
And when we add to that the Testamony of the earlychurch fathers, nonfathers, schizmatics, and heretics that were chillists, we see that they too understood it to be 666 and not 616........and they did it by using greek letters.......not hebrew ones.
Don't forget that the Apostles mostly used a greek Old Testament, and they mostly wrote the New Testament in Greek as well, and so they were very comfortable in using greek.