A String always has to have the number of characters as the first element in the String.
In the PLC-5 and SLC-500, the first Word was the length, and then the characters were packed into subsequent Words two at a time.
In ControlLogix, the first DINT is the .Length sub-element, and the characters are put into a SINT[x] array as the .Data sub-elements.
The easiest thing to do would be to copy those DINTs into an actual String datatype tag, and give it a fixed Length.