What passages of the LDS edition of the KJV and other authoritative writings and recorded teachings of the LDS support the teaching that Jesus is Jehovah of the Old Testament?
Jesus has many titles. We know him by "Son of Man", "Savior", "I AM", "The Word", "Christ", "Lamb of God", and so forth. "Jehovah" is the title give to the pre-moral Jesus Christ.
In the book of Isaiah, we read the words of Jehovah:
41:14 Fear not, thou worm Jacob, and ye men of Israel; I will help thee, saith the Lord, and thy redeemer, the Holy One of Israel.
44:6 Thus saith the Lord the King of Israel, and his redeemer the Lord of hosts; I am the first, and I am the last; and beside me there is no God.
Notice how "Redeemer" is used as He speaks or is spoken about. Jesus Christ is our Redeemer, and Jehovah mentions he is or will be the "Redeemer"
In the Book of John 8:58-59 we read:
58 Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am.
59 Then took they up stones to cast at him: but Jesus hid himself, and went out of the temple, going through the midst of them, and so passed by.
Here the Jews recognized that Christ confessed or claimed to be "I AM" who is Jehovah of the Old Testament. This they considered blasphemy and set out to stone him.
In addition to Biblical evidence that Jesus is Jehovah, we have prophets in the Book of Mormon who declared it. Abinidi was one (Mosiah 13:33-34).
33 For behold, did not Moses prophesy unto them concerning the coming of the Messiah, and that God should redeem his people? Yea, and even all the prophets who have prophesied ever since the world began—have they not spoken more or less concerning these things?
34 Have they not said that God himself should come down among the children of men, and take upon him the form of man, and go forth in mighty power upon the face of the earth?
The God of Moses was Jehovah, the God of the Old Testament. Here we see that Moses declared that God (Jehovah) would come to earth and "go forth in mighty power."
The very last verse in the Book of Mormon talks about coming before the judgement bar of Jehovah. But right before it (verses 32-33) speaks of coming to Christ and becoming perfected in him.
34 And now I bid unto all, farewell. I soon go to rest in the paradise of God, until my spirit and body shall again reunite, and I am brought forth triumphant through the air, to meet you before the pleasing bar of the great Jehovah, the Eternal Judge of both quick and dead. Amen.
We know from Isaiah 33:22 that Christ is our judge. This is another example of how Jesus and Jehovah are the same person.
Beyond The Book of Mormon we have modern revelation found in Doctrine and Covenants 110:3-4
3 His eyes were as a flame of fire; the hair of his head was white like the pure snow; his countenance shone above the brightness of the sun; and his voice was as the sound of the rushing of great waters, even the voice of Jehovah, saying:
4 I am the first and the last; I am he who liveth, I am he who was slain; I am your advocate with the Father.
Jehovah (Jesus Christ) appeared to Joseph Smith and others in the Kirtland Temple.
I am he who was slain is a good indication Jehovah is Christ who was slain for the sins of the world.
For more explanation here is a bit from the LDS Old Testament Student Manual