700c or 27" is irrelevant. Reach is reach, and is set by the maker based on tire section and/or fender clearance considerations. The only time the wheel size makes a difference is if you change from one to the other in the same fork, in which case the reach will change by 4mm in either direction (shorter going from 700c to 27") If the OP has a 27" wheel he already has the shorter possibility, so it's simply a matter of buying the right brake, modern or vintage.