One other thing that I failed to mention - the two 1' x 4' boards pass each pass through a dedicated cut out made into the superior horizontal board. It doesn't appear necessary to have two holes for one would likely suffice. At first glance, the portion of wood separating the two holes looks as if it provides the impetus for the "jaws" of the vise to open. In reality, it is the springiness of the two 1' x 4's that provides the force to spread the vise.
That said, if I attempt to make the above proposed change, the two 1' x 4's will pass through a common hole with a slightly smaller outside deminsion.