Here the 1830 typesetter substituted were for wax. The 1908 RLDS edition and the 1981 LDS edition correctly restored the original wax. It is worth noting here that the original reading is consistent throughout the Book of Mormon text: there are six other occurrences of “to wax strong in the land” but none of “to be strong in the land”:
Notice that there are two other examples of “multiply and wax strong” (in Alma 62:48 and Helaman 6:12).
Summary: Maintain in Alma 50:18 the original verb wax in the phrase “wax strong in the land”, the systematic expression in the text.