I'll second faustus, at least relative to old centerpulls. Mafac Racers with modern Tektro levers and Koolstop pads on a 27" to 700c conversion will give you big time stopping power. (Those brakes are still easy to find and cheap. A little play in the pivots is no big deal. They stop just fine even with large amounts of play. I've never replaced a pair because the stopping suffered. I find every 10-15 years a newer flashier pair appears and I put them on.)

When I had my Peter Mooney built, I had the canti bosses located halfway between 27" and 700c. When I ordered the bike, I wasn't sure what the wheel standard was going to be or what would be available, perhaps in backwoods Maine. As it happened the bike has never seen anything but 700c. The cantis have worked just fine.

Ben