To check the frame for square.
Choose a level, smooth surface to work on. A garage floor will work well, even though most slope.
Support the truck frame by jack stands with the frame as level as possible. It does not have to be perfect, just close at this point.
Find a factory hole at each end of the frame and the corresponding hole on the opposite side. They should be as close to the end as possible. These will be your reference points.
Drop a plumb bob from each hole and mark the floor. You should find that the points on the floor are the same distance apart from front to back and from side to side (if I remember correctly the 57 frame rails are parallel). It is also a good idea to drop the plumb bob at a mid-point on the frame to check for rail straightness.
Now measure the points on the floor, corner to corner. This will give you an idea how much your frame is out of square. Remember that the out of squareness is 1/2 of difference between the long dimension and the short dimension. In your example above, 5/16 longer one way would be out of square 5/32.
