The original manuscript twice uses the modal should in this verse. The 1837 edition changed the first should to shall, but this change was probably a typo rather than due to intentional editing since later the verse continues to use should (“but their garments should be made white”). A similar use of should after the verb say is found near the end of the Book of Mormon:
Both passages (at least originally) have “hath said” followed by should rather than shall, so there is textually nothing wrong with having should in a prophetic expression dealing with the future. For further discussion of the variation between shall and should, see under Jacob 7:9 as well as more generally under modal verbs in volume 3.
Summary: Restore in Alma 34:36 the original modal should, which agrees with the should in the following clause as well as with the similar construction in Mormon 5:9.