Just to let you know, there were two McCully's listed in the ranks of Porter's Regiment (AKA 1st Northeast Missouri Cavalry - CSA) according to the microfilm of the service records from NARA of that Regiment.

Joseph McCully, 2nd Sgt
William McCully, Private.

Both signed up on July 25, 1862 in Schuyler County in Company H, Capt James Leeper. William McCully appears to have been captured.

In supporting this capture, Joanne Chiles Eakin's book, "Missouri Prisoners of War" states that William McCuley was confined at Alton and released on June 25, 1865.