I have a single bike frame that someone respaced to 130mm that may illustrate a possible problem with the rod and washer approach. The left stay was not moved at all and the right side stay was bent to make the dropout spacing 130mm. The problem is that the right side stay is dimpled for chain ring clearance and therefore less able to resist outward force than the non drive stay. Apply equal force to both stays and only the drive stay gets bent.

Using a lever like a 2 x 4 will require a lot of power and courage. I have tried it on a single and was surprised how hard it was to get the frame to move. It does however allow for moving each half independently to keep the frame in alignment.

Regardless of the method bending the frame is risky business. I ended up giving up on my single bike frame because I decided I would rather have it the way it was than risk ruining it.