Isaiah 48:8 (King James Bible) for I knew that thou wouldest deal very treacherously
Here the 1830 typesetter replaced the wouldst of the manuscripts with the spelling wouldest, the King James Bible’s form of the word. The typesetter may have referred to his own Bible to make this change to -est in 1 Nephi 20:8 (as also for the word shouldst in 1 Nephi 21:6). Later editions have reverted to the original wouldst. For a complete discussion, see 1 Nephi 20:5.
Summary: Maintain wouldst in 1 Nephi 20:8 since it is the reading in both manuscripts.