Looking at the Montani transcription in the St. Gregory Hymnal as well as that of the ST. Pius X Hymnal. Leaning toward the St. Gregory version because as a congregation we've been singing it off and on for a number of years...

I understand. I'm not necessarily adverse to using accompaniment lines instead of assigned voice lines if it seems to "make sense" musically. We've done it any number of times using hymns that are "old standards", with pretty decent success, but this case in the St. Gregory Hymnal, does make it challenging. It is easier in the St Pius X Hymnal but it would be nice if I could find the voice parts. I have heard a polyphony voice setting on a recording, but the recording doesn't match either of these written scores. That is the problem and why I'm on this hunt. The recording was done on a CD by Robert and Robin Kochis, the CD entitled Ascension. The choral group is labelled as "the Donald Brinegar Singers". No luck in tracking them down either. So hey, I'm open to suggestions. If someone knows of a different score which contains a multiple voice setting, that would be great as well.

There appears to be a "choral arrangement" by Richard Proulx in the GIA catalogue.

I found the Bartolucci score online: http://www.contemponet.com/shop/product_info.php?products_id=936

Latona has a version combining Gloria VIII with polyphony by Cima: https://www.stantons.com/sheet-music/title/gloris-in-excelsis-deo/MSM80-110/

It's very nice

There is also the setting by Richard Rice.

http://www.ccwatershed.org/blog/2015/jun/22/richard-rice-kyrie-mass-viii/