I noticed that frame has rocker dropouts, do you have them adjusted all the way back? It looks like you could get a few more millimeters per side from more dimpling, but not enough to get a 42 in there. If it were mine and I could find a way to fixture it on the mill, I would cut out a section of the stays with a hole saw and weld in a section of heavy walled tube. It could also be done without the hole saw, but would definitely take longer to get a nice fit for welding in the section of tube.
This is what I do to fit a 3" tire on a frame with a 73mm bottom bracket. Something similar would work on your frame with a smaller diameter section of tube welded into your existing stays. Finding someone willing to do it for a reasonable price is another matter.
