Bear with me for a potentially stupid question. Theoretically, given a typical vintage road bike with a steel frame, sidepull brakes, friction shifting, and 700c wheels, *exactly* why couldn't you put 26" wheels on it?
By 26" wheels, I mean 559 mm mountain bike sized, not 650B or 650c wheels. I can see a problem with brake reach, but that could be solved with extra-long brake calipers. Is there a problem with the spacing of the dropouts? Where, exactly, are the incompatibilities?