The Eight you are speaking about was part of a Mahler Festival held during the 1976/77 Season of the New York Philharmonic.
It was broadcast on October, 9, 1976 and, according to my source, a copy should exist.
Place: Carnegie Hall
Carol Neblett, soprano
Teresa Zylis-Gara, soprano
Kathleen Battle, soprano
Lili Chookasian, contralto
Gwendolyn Killebrew, mezzo-soprano
Kenneth Riegel tenor
Michael Devlin, baritone
Donald McIntyre, baritone
Westminster Choir
Boys' Choir from the Little Chrch around the Corner
Trinity School Boys' ChoirĂ¹The Brooklyn Boys' Chorus
New York Philharmonic
James Levine
(source: Booklet "Mahler in New York", p. 190. This booklet is part of the amazing 12 CDs New Philharmonic set "The Mahler boradcast 1948-1982)
Regards,
Luca