Isaiah 49:6 (King James Bible) it is a light thing that thou shouldest be my servant
As noted in the discussion for 1 Nephi 20:5, the original text of the Book of Mormon favors the one-syllable shouldst over the two-syllable shouldest. In each case, we let the earliest textual sources determine the reading. In this instance, we have shouldst in the original manuscript, whereas the King James Bible has shouldest. The 1830 compositor set shouldest, probably by reference to the Bible he was using to check the Isaiah text. He made the same change for wouldst in 1 Nephi 20:8.
Summary: Maintain shouldst in 1 Nephi 21:6, the reading of the original manuscript.