Why an effective ironclad ram required a purpose-built hull

Could a purpose-built ram have changed an early ironclad battle such as Hampton Roads?

Potentially, but an effective ram required the whole hull to be shaped and strengthened for the collision.

A better ram could have changed an early ironclad battle such as Hampton Roads. The design burden would extend through the entire ship. Ramming loads travel through the hull, so the bow, structure, and overall form must be arranged for the impact. A ram was an integrated element of naval architecture rather than a bolt-on weapon.