Here we have an obvious typo in the 1892 RLDS edition; the replacement of long by along is probably due to the preceding all. As expected, there is no usage to support the use of along rather than long with “all the day”. The 1908 RLDS edition restored the correct long to the RLDS text. There are 12 occurrences of “all the day long” in the Book of Mormon, plus 13 occurrences in the King James Bible.
Summary: Maintain the use of “all the day long” in Mosiah 12:4 and elsewhere.