Yes, it was indeed Brenton's translation that I alluded to, and you are quite right to mention Hebrews.

For now (I need to check something else), a note in the NAB for Heb 10:5 clears it up some:

"NAB Notes (Heb 10:5) [5-7] A passage from Psalm 40:7-9 is placed in the mouth of the Son at his incarnation. As usual, the author follows the Septuagint text. There is a notable difference in Hebrews 10:5 (Psalm 40:6), where the Masoretic text reads "ears you have dug for me" ("ears open to obedience you gave me," NAB), but most Septuagint manuscripts have "a body you prepared for me," a reading obviously more suited to the interpretation of Hebrews."

The version of the LXX that I consulted (edited by Alfred Rahlfs, 1935), as I noted, has this as being similar to the MT (Masoretic text).