The soldiers you cited are two different men.

The man who served in Co. L, 2nd Arkansas Cavalry, U.S. Volunteers, was William B. Wilbourn, born c1842 in Pulaski County, Arkansas, and residing in Franklin County, Arkansas, when the war began. He had previously served in the Confederate army, in Williamson's Arkansas Battalion and later in the 15th Northwest Arkansas Infantry.

The man who served in Co. C, 3rd Arkansas Cavalry, U.S. Volunteers, was William Welborn, born c1828 in Conway County, Arkansas, and residing in Conway County when the war began. He had previously served in the Confederate army, in the 1st Arkansas Mounted Rifles and later in the 3rd Arkansas Cavalry (CSA).

As for John Wilburn in U.S. service, there was a John W. Wilburn, born c1827 in Georgia, who enlisted in Co. H, 1st Arkansas Infantry, U.S. Volunteers, at Fayetteville, Arkansas, on March 10, 1863.