According to Canney's "The Old Steam Navy: Frigartes, Sloops, and Gunboats, 1815-1885" (Nuois, aval Institute Press), the 11" Dahlgrens were onboard from the beginning, the nine inch Dahlgrens likley coming onboard during her 1871-1873 refit in place of the 32 pounder Dahlgrens and the rifled Dahlgren on the focs'l.
The line under the yardarms is likely the foot rope that sailors used to make their way out on the yard to take in or lower sails (holding on to the yard with their hands and stepping sideways). There should be pictures of builds of the Kearsarge on this site and the Model Warships Gallery. You can also check the USN Naval History Site (http://www.history.navy.mil/branches/org11-2.htm) for information. The Kearsarge was not a standalone ship, but one of a group of sloops authorized in 1858 that were more of less similar with the Kearsarge and Mohican being close in design - so any information you find on the Mohican would probably help with the Kearsarge.
A number of folks have noted that the kit was based on the plans of the Kearsarge AFTER the Civil War so that the kit does not depict the ship as she looked in 1864 for her engagement with the Alabama, so you might want to look at pictures and plan from 1870 or so onwards.
Other sloops in the "class" included the Wyoming, Tuscarora, Iroquois, Oneida, Wachusett, and the Dacotah.
Good luck!