I'd echo Keithglos' remarks. I did a similar operation to re-space my Witcomb. Simply widening the back can leave the rear dropouts out of parallel. You may need to bend the rear ends inwards a little so they are parallel all along.
I used the lever approach (Sheldon Brown) working on each side to ensure that the widening was equal. The right stay can bend more easily if it has the chainring depression.
I used the lever approach (Sheldon Brown) working on each side to ensure that the widening was equal. The right stay can bend more easily if it has the chainring depression.