The best staring point for research is place of residence in 1860. Since many men from the same state could have the name Joshua Preston or William Preston, this is the best way to focus your search. Otherwise you will be guessing.

Joshua and William Preston appear on the 1860 census of Monroe County AL. Joshua Preston served as Sergeant Major of the 42nd Alabama Regt. He transferred to this command from Co H, 17th Alabama Regt. Being an older man, William B. Preston joined Co "F", 4th Alabama Reserves, and served as 2nd Sergeant. In 1887 his widow Rebecca applied for a pension in his name.