Does your number of Arkansans in Union regiments take into account all the Arkansans who served in federal units from Missouri, Kansas, etc? Just curious. I would "guestimate" there to be about a thousand such men, maybe a more. Of course given the sources it is difficult to determine exact numbers.
I recently did a statistical sampling of the 2nd Arkansas Cavalry (US) and identified 82% of the sample as residing in Arkansas at the time of the 1860 census. I think the overall number is probably a little higher than that as several of the men moved into the state after the census was taken. About 10% of the men in the unit were Missourians. The remainder were Texans, Native Americans, and others. A very interesting unit, if I do say so.