The regimental capsule history should help.
I am unable to locate the one letter that I have from the 5th Miss. You may find more information by contacting the Miss. Dept. of Archives and history. Sometimes they have special collections for a regiment. Also look at the records of Colonels, and generals who survived the war and check their home state archives. Mississippi State and Ole Miss. may also have records. Check the other units in their Brigade as well. You are starting where I started 10 years ago. You have to dig and dig and find others to help with the research. Good luck and keep us posted.