If you're actually going to permanently bend the stays to the new wider spacing then I'd suggest doing it the way shown and described on Sheldon Brown's website in the Coldsetting article. If you're doing this you will want to make sure that you bend each side out by the same amount. And just grabbing the dropouts and heaving on them or using an MTB axle and nuts does not guarantee that both sides will flex and bend the same. Far from it in fact. On the other hand by using the method shown in Sheldon's write up you will bend to measure one side at a time while checking the opening. The first side being bent until the spacing in increased by 1/2 the desired amount and then the other side being bent out to finish the job.