In a letter, Cather says that she relied on Howlett for her understanding of her "two priests" in Death Comes for the Archbishop.
In Death Comes for the Archbishop, John March (22) reports that Father Joseph Machebeuf, the prototype for Joseph Vaillant, told the story of the white mules to Father Gabriel Ussel and that Vaillant's motto "rest in action" is an oft-repeated phrase in The Life of the Right Reverend Joseph P. Machebeuf.
In "Historical Essay," there is a reference for Death Comes for the Archbishop; see Cather's November 23, 1927 letter to the Commonweal.