With care it shouldnt be a problem, i opened a 531 frame from 126 to 135 using the lump of wood method. No issues with dropout or hanger alignment, the only thing that concerned me was the rear brake bridge which is a drilled tube on my Clubman. Its quite slender and i became concerned it was going to seperate from the stays so i used a big jubilee clip to clamp it all together while i was bending, Probably quite uneccessary, but you know how it is when a thought gets in your head,