Running with the majority percentages that most pension applications from Montgomery County, Arkansas were for Confederate service, A possibility is the 6th Arkansas Cavalry CS, sometimes known as the 1st Arkansas Cavalry, but most commonly known as Monroe's Arkansas Cavalry. It was also sometimes referred to as "O'Neill's Arkansas Cavalry" in 1864 and 65.

Lt. Col. James M. O'Neill was commanding the 6th Arkansas Cavalry and killed at the Battle of Marks Mill April 25th, 1864. In pension records, well after the war, Col O'Neill Regiment was sometimes listed by the applicants as "McNeill's Regiment". However, we are not able to confirm confederate service after 1864, because the Confederates in Arkansas were unable to keep records after February 1864 because of lack of paper. So there is no record of those who joined after that time.

There may be a back door method which you might check. His pension application should have the name of someone who atested to James Helms service for the pension to be approve. You may be able to check the names of that person against the muster roll records to determine if indeed what unit James Helms served in.