Here are some clues that seem to point to organization being at Devall's Buff:

ROWLAND B. SMITH CIVIL WAR LETTERS

March 26, 1862 Little Rock-Description of Gen Van Dorn's Little Rock visit-he arrived night before last; We drew up in line to receive him. The question was asked about guns, and he said he had 2000 fine Enfields with saber bayonets and from the looks of us he did not think we would disgrace them. He says we will not organize our regt here as there are a great many men hanging around looking for appointments. I think him right as there are enough men around town in fine uniform coats and hats to make a regt.
Van Dorn has ordered Gen Rust to take us to Jacksonport or Devall’s Bluff near there and we will organize there.
The war is one large swindling machine-corruption in Army departments, etc.
Seated in my tent as I write you- We are being drilled 9 hours a day, every day by our drillmaster Col Platoz of New Orleans. He is an excellent drillmaster. Some say our Co is as well drilled as other Co’s who have been here 3 weeks.
Write to me care of Capt Southerland, Rust’s Brigade

Into the Mouth of the Cannon A Historical Biography of the 18th Arkansas by Robert E Reynolds

Co H sworn in at Little Rock then trained to Devall's Bluff where organized into a regiment.

Eleanor S. Brockenbrough Library
The Museum of the Confederacy
Major General Earl Van Dorn (Commander of the Trans-Mississippi District, Army of the West) Letter Book, 1862

3/25 HQs Trans Miss Dist. Little Rock, Maury addressed to Gen Rust, with a copy to Col J P Morgan- -I am instructed by Gen Van Dorn, Cdg this Dist.., to say that he desires you to assume command of the troops in the region of country about the River Ark, from Clarksville to the mouth of the river & about White River from Des Arc to its mouth.

-You will proceed at once to organizing Reg’ts from the Co’s organized. The Co’s now at Little Rock will be sent to Devall’s [sic] Bluff under the command of Col. James P. Major, where the organization will take place. The Co’s North of the Ark River, that were formerly ordered to proceed under the command of Col. Major to Jacksonport, will proceed under his command to Des Arc, where they will be organized into Reg’ts & Bns under your direction.
-The Co’s organized under the call of Gov. Rector for the Confederate Service, & heretofore directed to proceed to Jacksonport under the control & supervision of Col James. P. Major, are hereby directed to change their march to Des Arc on White River. Col. Major will make the necessary arrangements to carry out these instructions.
-The Co’s now at Little Rock will proceed at once to Devall’s [sic] Bluff, where they will go into camp, be assessed, and await orders. Col. Major will superintend & control their movement also, & will make the necessary measures for the prompt execution of their orders, & for the proper care of the men.
-Gen. Rust, after assuming command as heretofore directed, will cause all the Ark Troops who may report to him to be organized into Reg’ts after their arrival on White River.