I am trying to find out where R.Moshe Feinstein wrote that a convert can become a rav even a secondary source would be good.
ב"ה Shalom We have no more primary source that the Talmud. The Talmud say that the ancestors of Sancheriv taught Torah to the public. (Gittin 57b). the Talmud then says that the ancestors referred to are none other than Shma'aya and Avtalion who are of the early Tana'im (scholars of the Mishna) who had both converted to Judaism. We see from this that a convert can attain the highest levels of teaching Torah. Although, a convert is considered completely Jewish, among the few limitations which exist, is that he may not be appointed as a Dayan- a Rabbinical judge to deal with monetary issues since he is restricted from assuming responsibilities of authority and a Dayan is considered a function of authority (Talmud Yevamot 45b) Some say this restriction does not apply if the parties involved accepted the ger to be the judge and there are regulations which apply. All the best שנה טובה וגמר חתימה טובה