(Teshuva written in 1969 to Rav Yechiel Michel Twerski of Milwaukee)
In tackling the issue of different havarot, Rav Moshe begins by stating that a person should not stray from his family's havara. He then points out that although only one havara can actually be the correct one, namely the one that Hashem uses and that was used to give the Torah, the fact is that all are acceptable. Were this not to be the case, then every time a chalitza is done it would have to be done using all possible havarot just to ensure that at least one was correct. Rav Moshe further points out that even within havara Ashkenazit there are a variety of options, and thus, while only one can actually be correct, they all have to be acceptable on some level.
The final paragraph of this teshuva both asserts the primacy of Hebrew - however it is pronounced - as the language appropriate for prayers, and notes that in America, where havara Ashkenazit is predominant, one should use and teach that pronounciation.
[My note - I am not sure if Rav Moshe is actually referring to all of America in his last paragraph, or only to his community. Obviously, there is a large Sephardic community that would disagree with his assessment. Additionally, Rav Moshe does not write what distinguishes one havara from another - is it the taf/saf difference? The pronounciation of the kamatz? The attention to proper mil'eil/mil'ra? Saying oy instead of oh?]