A 27" bike with long reach brakes from the factory is a really nice frame to convert to 700C as it most likely allows for a wider tire and fenders with the smaller 700C rims. How wide a tire can fit with fenders depends on the frame, but typically it's the next size up (or 2 sizes up) from the widest that originally fit. A 27" bike with 'racer' dimensions might work going to 650B if the frame was originally sized to fit really skinny tires, no fenders, and used short reach brakes with the pads at the top - but that's not a common frame design for a vintage 27" bike.
Originally Posted by
Speed2XS
I may have to go this route, but have been trying to avoid it as I don't have the skill to fabricate as nice a piece as the production drop-bolt. What my goal ultimately is, is a 27" frame with 650x2.0 knobbies. I have the frame for it, but no brake solution yet.