The preference would be 700C just because of the aviaiable tire and rim choices. Finding a hooked bead, 27" rim may be hard but I think Sun and Velocity still have a model. It doesn't look like the rear caliper has enough adjustment to accommodate the extra 4mm drop required for the 700C. However, the Dia-Compe were availble in a 750 model, long reach version (60-78mm), which are not that hard to find. Then again, you may want to consider updating the brakes to a modern, long reach, dual pivot design. They perform much better. At minimum, consider new cables, teflon lined housing and a good brake pad. It will make a noticeable difference.