The original text of the Book of Mormon tends to use the subjunctive more frequently than what speakers of modern English expect. A similar example is found later on in this chapter:
In 1 Nephi 17:18, the subjunctive were occurs in a negative context; in 1 Nephi 17:41, were is found in a conditional context. Both these examples of were were replaced by was in the 1830 edition, perhaps intentionally. The critical text will restore these instances of the subjunctive were. For a complete analysis of the editing involving the subjunctive, see under mood in volume 3.
The passage in 1 Nephi 17:18 also shows a minor error that entered the 1892 RLDS edition. Under the influence of the preceding main clause (“for they did not believe that I could build a ship”), the modal would was replaced by did, thus increasing the parallelism with the following clausal conjunct (“neither did they believe that I was instructed of the Lord”). This error was removed in the subsequent RLDS edition (1908).
Summary: Maintain the use of the subjunctive were whenever it is supported by the earliest textual sources, as here in 1 Nephi 17:18 and also in 1 Nephi 17:41.