Translation:We had become best friends that year.
you can't use better since it would be as if there was an improvement in their friendship. In this case though, Migliori amiche means best friends. Example: "Lei è la mia migliore amica" She's my best friend while "Siamo amici migliori adesso" We're better friends now. Hope it helps.
"Better friends" accepted 29 Dec 2017. I thought I was taking a chance there.
One never knows how much input from google translate is worth, but it translated migliori amiche as "best friends" and * più migliori amiche* as "better friends - which seems backwards. I wouldn't take that to any degree of certainty, though.
I think it's one of those word order things, like the difference between "old friends" and friends who happen to be old: migliore would have to be after amici. But see the comment from @lyssnerskan below: "For "better friends", natives usually use "piú amici" and not "migliori amici" "
Llaaama: Yes, it's 'diventatE' because it refers to a group of women or girls. The form "diventate" is NOT the 2nd person plural present tense, it's rather the past participle which has to agree with the subject in verbs using 'essere' as you have here. So the "We" of the sentence must be women or girls, as you can see from the plural FEMININE form of 'friends" 'amiche" If it were 'diventati" then the subject would have had to be either all men or a mixed group of men and women, boys and girls, etc.
English is weird in the relationship of simple past to present perfect and past perfect. "We became" definitely can mean exactly the same thing as "we have become" (present perfect), but it can also be used as a substitute for "we had became" - it all depends on the temporal sense of the narrative surrounding the statement use some conjugation of "to become".
In the days following the Ice Age, the European Neanderthals became extinct. By the end of the Ice Age they had become extinct. Anyone living at the end of the Ice Age would say that they became extinct recently, and as of that moment in time have become extinct just a few years before.