A couple other things that occur to me:

Bead seat diameter is, technically, where the tire bead hook on the inside of the rim, and it is not exactly where centerline of brake shoe needs to be; on some old Mavic 700C rims I have readily available (622 BSD) the brake shoes sit at about 312 or 313 mm from axle centerline. But if you plan brake reach for the BSD, you'll probably be okay.

There are longer reach brake calipers available for replacement use on older bikes (where "standard" reach was in the 47-57 mm range) and also for touring purposes (need for fenders, use of larger diameter tires). Tektro is one company making these now (models 621A and R556, and eBay has plenty of vintage brakes for sale, both road and early mountain bike. Try this link:
http://www.velo-orange.com/brakes.html
The first number is a 47-57 reach, and the second is 55-73.