That beautiful ballad was "I Do Not Know a Day I Did Not Love You," sung in the show by Walter Willison, who received a Tony nomination. Here's Willison, 36 years later, performing the song at the 2006 Theatre World Awards.
He does a terrific job, though over the years this has become enough of a signature song for him that he now seems to milk every syllable out of it. That said, once he gets into the song, he does seem to let the music and momentum of the song take over, and it moves along much better.
Willison played the youngest son, Japheth. In the song, he finally acknowledges that he's in love with the wife of one of his brothers.
That's Danny Kaye as Noah on the far left, and Willison the third from left. The woman he's in love with is Rachel, played by Tricia O'Neil at his left elbow.