A great sefer for research on minhogim, among other things, is the sefer כתר שם טוב, penned by the Chacham R. Shem Tov Gaguine, a great Sepharadic talmid chochom, who was a Rav in England, among other places. Some of the volumes are available via the great Hebrewbooks.org website.
I have recently become more acquainted with it, and have found some very interesting things there.
I have seen a number of cases, where, surprisingly, the minhogim of Spanish and Portugese Sepharadic communities discussed there are the same as those of Yekkes. The Western European Sepharadim are a community parallel to the Yekkes among Ashkenazim in a way and to a degree, in that they preserved certain old Spanish minhogim like the Yekkes have preserved old Ashkenaz ones. So that points to old traditions of ספרד and אשכנז being in accordance with regard to those matters.
I assume that as time goes on I may אי”ה find even more such correspondences, but for now I will share a few such cases.
2) The section starting רבונו של עולם הריני מוחל is not recited in the beginning of קריאת שמע על המטה (see footnote toward bottom of page).
4) Tefillas Rav is not said in Rosh Chodesh bentching (footnotes).
5) It states that the old minhog in London was that on Shabbos it was announced who would say kaddish in the coming week (footnotes near bottom of page) (as only one person said kaddish at a time, as in מנהג אשכנז, as we have been discussing recently. So we see that Sepharadim practiced the singular kaddish in the old days as well).
This has bearing also with regard to early American Sepharadim and their congregations, such as the Spanish and Portugese Synagogue in NY, where some, if not all of the minhagim of the Western European Sepharadim are followed as well.
So we see that despite what people may think, Sepharadic and Ashkenazic practices are not always so different, and there can be surprising and significant convergences. After all, we are brothers, of course.
(There are other minhogim which a broader range of Sepharadim and Yekkes share as well, which רבש”ה spoke about in shiurim last time he was in the USA, which we may discuss in the future, אי”ה).