I don't recommend this approach. 6mm is a large spread. It will change the angle of the dropouts relative to the locknuts by about 0.5 degrees. Even though this sounds small, clamping the wheel into non-parallel dropouts induces stress in the rear axle, making the axle more susceptible to bending.

Either get the reat triangle cold set or use a narrow, six speed freewheel, as previously suggested.