Add Chord Names (Guitar Chords) Over Popular Traditional Catholic Gregorian Chants' Hymn Sheet JPGs

Gregorian chant traditionally only has lead-line notation. It is simpler than modern notation. I want to be able to give basic accompaniment on the organ, by playing the chords, one-handed, to accompany the choir singing. I am not a trained musician and can't sight-read. Modern notation is way out of my league, hence this request. - I attach an example of what I had done previously, with the files I want annotated now. - This job would suit a pianist who can work out what chords should accompany the hymn's melodies and can type these into a program like Gimp (free) or Photoshop. You can probably 'cheat' and save time by looking up the modern notation versions of 'Missa VIII' and 'Credo III', 'Missa de Angelis', and get the chords from those. Summary: I want Missa VIII and Credo III annotated with suitable accompaniment chords. Thank you.