1 Nephi 19

Nephi learned from the brass plates -

"...the God of our fathers, who were led out of Egypt, out of bondage, and also were preserved in the wilderness by him, yea, the God of Abraham, and of Isaac, and the God of Jacob, yieldeth himself, according to the words of the angel, as a man, into the hands of wicked men, to be lifted up, according to the words of Zenock, and to be crucified, according to the words of Neum, and to be buried in a sepulchre, according to the words of Zenos, which he spake concerning the three days of darkness, which should be a sign given of his death unto those who should inhabit the isles of the sea, more especially given unto those who are of the house of Israel."  (1 Nephi 19:10)

First and foremost, ancient prophets understood the "God" of the Old Testament was the Savior.  And second, it becomes clear that when ancient prophets were very clear, easily to be understood, in their prophecies that their words were not included in our scriptures today.