Well, I did it... I used basically the method that Tuz suggested- a vise with a short little torpedo of steel. It worked a charm- it didn't even mar the paint, and added about a quarter inch of clearance to each side of the tires. It did make the chainstays pull together as Frank said it would, but I was planning on cold-setting the frame from 126 to 130 anyway- I just had to cold set it a little more.

I don't think I'd try it if the chainstays were chrome-moly.