If the turnbuckle is not long enough to span across a frame simply tack some bar to it. The photo shows a turnbuckle that is in regular service and paid for itself many times over. Scrap bar is tacked on the ends from the last time it was used.
When tack welded across the corner of a frame the turnbuckle can be used to push or pull the corner square. Keep in mind that the corner must be lightly tacked so that it will hinge.
Once square welding can commence and the turnbuckle can be removed after everyting is cooled down.