The 1852 LDS edition changed the singular heart is to the plural hearts are, undoubtedly because in the following clause the plural pronoun they refers to “your hearts”. The 1858 Wright edition and the 1953 RLDS edition also made this grammatical change. There is one other reference in the Book of Mormon to the heart swelling, although not with pride but with thanksgiving: “yea a man whose heart did swell with thanksgiving to his God” (Alma 48:12).
Elsewhere in the text, there is an occasional use of the singular heart in contexts where readers expect the plural. For some examples, plus discussion, see under Alma 32:28. As explained there, the critical text will restore all instances of the singular heart when supported by the earliest textual sources, even when the context indicates that the plural is expected. Thus here in Helaman 13:22, the critical text will restore the singular heart is, despite its blatant conflict in number with the following plural pronoun they.
Summary: Restore the original singular heart is in Helaman 13:22; the original text allowed cases of singular heart despite the expectation of the plural hearts in the larger passage.