In my searchings I uncovered that blake's Spring and all his Songs of Innoncence were set to music for men's chorus by Adler in 1982! In some places the text is printed with what seems to be an original unedited version (the audacity to edit Blake!) where the ending couplet (and refrain) for each stanza goes like this, respectively:

Merrily, merrily, merrily,
To welcome in the year.

Merrily, merrily, merrily,
To welcome in the year.

Merrily, merrily, merrily,
We welcome in the year.