RickM, for 6 ft high sides, it would leave you <4 ft since some of the width would be taken up by the arch. You could put 2 or 3 layers of concrete block and put the sides in the holes of the blocks. That would give you another 3 to 4 ft in width with 6 ft height, more if it is arched. Would be lower on the sides and higher in the center.
