Jim, this man actually served in (old) Co. H, 23rd Arkansas Infantry, and you should find his service record there. After its capture at Port Hudson, the 23rd Arkansas was exchanged back in Arkansas, where it was reorganized and designated by the State Military Board as the 42nd Arkansas Mounted Infantry, though this designation does not seem to have been actually adopted by the regiment.
John Welch enlisted at Arkadelphia on March 6, 1862, by Capt. A. A. Pennington. He was wounded and captured at Corinth on October 4, 1862, and paroled at Bolivar, Tennessee, on October 13, 1862. He was captured at Port Hudson, Louisiana, on July 9, 1863; and paroled on July 10, 1863.