Jim --

If McClain somehow managed to survive the ordeal of a winter retreat with the Army of Tennessee, that would explain his presence in northeast Misissippi in January of 1865 --
http://www.historyofwar.org/articles/wars_tennessee.html

Forrest also brought survivors of his command into camp in the same area, but I don't understand a connection to the 40th Georgia Regt. Over the next few weeks able-bodied members of this regiment would have been prepared for transportation east into the Carolinas to fight Sherman. Forrest remained in Mississippi as part of General Richard Taylor's command.