I have spread a steel frame single to 130mm. I believe Sheldon Brown's web site has a page describing the 2x4 process, but I instead used a car jack between the stays. waynesulak raises legitimate concerns, but it worked fine for me, so I'm inclined to believe that the clamping mechanism described by WPH would work, particularly if the stays are braced so that the dimpling doesn't result in asymmetric bending.