The USS Powhatan could have a very distinct paint job according to the date. As you can see in this image the gun ports were not individually painted like the older naval sailing ships and do not show when they are closed. The side-wheel housing changed shape too; it was either square or rounded like above. However, a paint job could make the old side-wheel frigate look like any of Collins Line U.S. Mail Steamers like the Baltic, Arctic, or the Pacific Mail Steamship California etc.; all had the basic look, three mast, two mast behind the side-wheel houses, a single funnel forward of the side-wheels; foremast ahead of the funnel, and a bark or barque sail-plan.

Flying a British flag and repainting the ship would have had the effect of confusing both sides, British naval side-wheelers were rarely of this size, and I don't believe British Mail Steamers like the Trent (which almost fits the description) sailed in to Pensacola in the 1860s. So, it might not have fooled anyone.

David