The use of a length of 3/8"all-thread rod, two nuts and two large washers is a very old and well established method of spreading dropouts. I remember reading about this technique over 20 years ago and doing it twice on steel frames to make 126 mm frames accept 130 mm hubs with no forcing. It worked very well and, as far as I could measure, spread the stays evenly. I certainly found it easier to control than Sheldon's somewhat dramatic approach.