To answer my own question. I did a simple displacement calculation on the Monitor and Cannonicus using the known length, width, and draft. From this determined a hull shape adjustment factor, which takes into account the curved shape of the hull and the overhang of the armor, that I could multiply my rough displacement calculation against to get their known displacements.

Running the same calculation using the Passaic's length, width, and draft; and using the average of the hull shape adjustment factor from the Monitor and Cannonicus; I arrived at a displacement for the Passaic of 1450 tons.

Since the Passaic's hull shape and overhang were closer to the Monitor than the Cannonicus, I believe those sources that give Passaic's displacement as 1335 tons are correct.
1875 tons is too far off.