I feel your pain. I need new tires for my daughter's 51 Heron Touring, which is 26" wheels. I ended up ordering some Fairweather 26x1.75 from Blue Lug.
I could switch it to 650B. The brake pads would be just at the tops of the slots, and there would probably still be room for 38s. But, I worry the standover height might be more than my daughter is comfortable with.
Anybody out there with a 51cm Heron Touring that they've switched to 650B?
Ted Durant
Milwaukee, WI USA