I'm addressing both of your posts in this response.
The writer of Hebrews was quoting Psalm 40:6 and Psalm 139.
Psa 139:13 For thou hast possessed my reins: thou hast covered me in my mother's womb.
Psa 139:14 I will praise thee; for I am fearfully and wonderfully made: marvellous are thy works; and that my soul knoweth right well.
Psa 139:15 My substance was not hid from thee, when I was made in secret, and curiously wrought in the lowest parts of the earth.
Psa 139:16 Thine eyes did see my substance, yet being unperfect; and in thy book all my members were written, which in continuance were fashioned, when as yet there was none of them.
The body in Psalm 139 is the body of Christ... the church. This is the body the writer of Hebrews was talking about.
The Septuagint disagrees with KJV in several places, But you can't say the writer of Hebrews was quoting Deuteronomy just because it's in the Septuagint. I don't know what he was quoting, but I guarantee you it's hidden some where in the bible just like the body prepared for him is hidden.
What you said about Psalm 139 doesn't address the problem that the KJV doesn't match what Paul quoted. A study of church history will show that Jesus and the Apostles used the Septuagint as their OT, not the Masoretic text. The text the KJV is translated from is from around the 700's AD. There are quite a few places that the KJV OT differs from the NT. If it was the pure word of God it wouldn't wrongly quote the OT. The argument that the KJV is the pure word of God doesn't hold water when it's got errors.
New Testament
1 Peter 4:18 ( KJV ) [SUP]18[/SUP]And if the righteous scarcely be saved, where shall the ungodly and the sinner appear?
Peter quotes Proverbs 11:31
Masoretic text
Proverbs 11:31 ( KJV ) [SUP]31[/SUP]Behold, the righteous shall be recompensed in the earth: much more the wicked and the sinner.
Septuagint
Proverbs 11:31 If the righteous scarcely be saved, where shall the ungodly and the sinner appear?
New Testament
Matthew 15:7-9 ( KJV ) [SUP]7[/SUP]
Ye hypocrites, well did Esaias prophesy of you, saying,
[SUP]8[/SUP]This people draweth nigh unto me with their mouth, and honoureth me with their lips; but their heart is far from me. [SUP]9[/SUP]But in vain they do worship me, teaching for doctrines the commandments of men.
Here Jesus quotes from Isaiah 29:13
Masoretic text
Isaiah 29:13 ( KJV ) [SUP]13[/SUP]Wherefore the Lord said, Forasmuch as this people draw near
me with their mouth, and with their lips do honour me, but have removed their heart far from me, and their fear toward me is taught by the precept of men:
Septuagint,
Isaiah 29:13 And the Lord has said, This people draw nigh to me with their mouth, and they honour me with their lips, but their heart is far from me: but in vain do they worship me, teaching the commandments and doctrines of men.
New Testament
Matthew 21:16 ( KJV ) [SUP]16[/SUP]And said unto him, Hearest thou what these say? And Jesus saith unto them, Yea; have ye never read,
Out of the mouth of babes and sucklings thou hast perfected praise?
Masoretic text
Psalms 8:2 ( KJV ) [SUP]2 [/SUP]Out of the mouth of babes and sucklings hast thou ordained strength because of thine enemies, that thou mightest still the enemy and the avenger.
Septuagint,
Psalms 8:2 Out of the mouth of babes and sucklings hast thou perfected praise, because of thine enemies; that thou mightest put down the enemy and avenger.
There are more.