Minhag HaGra is to duchan every day. The Gra wanted to make that the minahg of his bet midrash but a fire broke out and he announced that he ddi not have the support of Heaven. Same when Rav Chaim Volozhiner decided to change the minhag. Howeevr, Rav Yisrael Salanter said that if he had agreed withthe Gra he woulkd have changed it fire or no fire. Rav Yaakov Kaminetzky said that RYS thought that the fire had been set by fanatics and neither the Gra or RCV wanted to make a fight of it. RYS did not care.
In any case, I do not understand why an Ashkenazi cannot duchan in a Sephardi shul as that is the minhag hamakom, especially if he davens there every day (see Iggerot Moshe Orach Chaim 3:5). Of course, he can make aliya as the standard Ashkenazi minhag in EY (except in the Galil and Haifa) is like the Gra.