If nothing works out well, you might consider looking for these Diacompe AGC 300 Aero Grand Compe brake calipers for your bike.
http://www.velobase.com/ViewComponen...=117&AbsPos=24
Notice how the top brake cable casing perch angles to one side. (to the right for the rear one). At least it might help keep the contortions of the brake cable and casing to a minimum.....

Good luck and tell us which solution you went for!

Chombi