Constitution remained a heavy frigate, not a disguised ship of the line

Were Constitution-class frigates built with third- or fourth-rate timbers on a frigate hull form?

No; they were unusually heavy frigates, but not framed strongly enough for the multiple gun decks of a third or fourth rate.

Constitution-class frigates did not use third- or fourth-rate structural scantlings. Their timbers were heavy for frigates, which contributed to durability and combat reputation, but the hull was still designed around a frigate's deck arrangement. Supporting multiple full gun decks with the weapons expected of a ship of the line would require substantially heavier and more expensive construction. The limits of congressional funding reinforce the design evidence against treating the class as a secretly rate-built hull.