I just picked up an 81 Schwinn Super Sport. It looks like it would make an awesome 650b conversion. You would need an early eighties one because they changed the geometry more towards racing. But it is fairly relaxed geometry, Champion #2 tubing, front and rear racks, triple chainring, Tange headset, Cyclone derailleurs, Superbe shifters.

Mine would probably need new brakes, but I just saw one from 83 (or 84) that had center-pull brakes that looked like they had plenty of reach.