Back to the OP's need to finish his conversion: I'd double check that the existing brakes can reach the new braking surfaces. It's only 4 mm more reach than with the 27s.

And there's a lot you can do with centerpulls to optimize their responsiveness and power: new outer cables of the correct length, with Teflon liners, squared -off ends and ferrules (I just like ferrules), new inner cables drawn for low friction, stiffer front cable hanger like a Velo-Orange, cleaned and lubed caliper mechanisms, cleaned springs with grease on the contact points, and the Salmon pads. New straddle wires with some time spent in finding the optimum straddle length /yoke height will get as much mechanical advantage out of your installation as can be had.