grammaticality - "[Noun] upon [noun]" — singular or plural?

I am copy-editing a manuscript in which the author has written the following sentence:

Rank upon rank of theologians has envisioned God the Father as the omniscient and omnipotent one.

"Rank upon rank" suggests plurality, so my instinct is to change "has" to "have". Am I right?

Alternatively, I'm tempted to suggest a more invasive change to avoid the awkwardness altogether. I suspect, however, that I personally do not understand how to handle countable and uncountable nouns with verbs, and I don't want to push this author unfairly toward a less creative sentence in the meantime.