The unit that you are searching for may be the 20th Alabama Infantry CSA. Company "G" was from Shelby County, AL. and was called "The Cahaba Valley Guards". They fought at Lookout Mountain and Missionary Ridge as well as numerous other well known campaigns. My Great Grandfather, Robert Finley Johnson was a member of Co. G. from it's muster in 1861 until he was wounded and captured at Nashville in 1864. If you think that this is the correct unit I can share some of it's history with you. I have a copy of the original muster of Co G. but due to some renovations I must search for the file. I will check for your William and let you know if he is listed as soon as I can.