Here the original text had the subjunctive were in a conditional context. Such usage is characteristic of older English. (For another example, see 1 Nephi 17:18.) In this instance, the 1830 compositor replaced the were with the indicative was, perhaps intentionally. The critical text will restore this case of the subjunctive were. For a complete analysis of the editing involving the subjunctive, see mood in volume 3.
It is also possible that the were is simply due to the preceding plural they in the relative clause that modifies the singular labor. Even so, the critical text will maintain the were of the earliest textual sources. (For further discussion of the influence of promixity, see subject-verb agreement in volume 3.)
Summary: Restore in 1 Nephi 17:41 the subjunctive were, the reading of the manuscripts; the original text uses the subjunctive more frequently than current English does.