Maybe I'll take some pictures of my frame later to clarify. I am using the re-issue Columbus SL (standard size tubes) with round oval round chainstays. It looks to me that you could dimple the stays to fit a 42 mm tire with reasonable clearance. The only problem I had with my bb shell was the angle from the seat tube to the chainstays; the bb was a little closer to the ground than I felt comfortable with so I blacksmithed the sockets. There is also the angle between the chainstays. Mountain bikes have problems with chainstay clearance, and there are all sorts of tricks that people use. They are trying to clear 2 1/2" tires though, that's around 17mm bigger than the tires you are proposing to use.