A more likely Voyage
By coming from China, and using "Great Arc" navigation (like the airlines do to make the distance as short as possible), the entire voyage is just over 9,000 miles. Because both wind and current in the North Pacific would be steadily in their favor, it would not be unreasonable for them to average 3 to 4 miles per hour. At 9,000 miles, that is between 2250 to 3000 hours which equals about 14 to 18 weeks (or less than 4 to 5 months). This is much more in keeping with the amount of ink the voyage portion of the narrative gets, and is much more reasonable for this group to have been able to do.
17,000 Miles (total from Saudi)
In summary, adequate resources, the shortest possible voyage, prevailing winds and currents, and Nephi's narrative all point to China as the old world Bountiful where they built their ship.
The Lord taught Nephi to build his boat, "not after the manner that was learned by men." (I Nephi 18:1-2). Was this the first Chinese junk ever built? Did the Chinese copy it? Who knows, the dates are close.
What is clear is that Nephi did build a boat, and they came to America.