well they make 650b x 38 so that might work for you.. granted not all brands will have that size. My smallest 650b tire is 42mm (Donnelly MSO) they are 43.5mm on 24mm wide internal rims. My WTB byways are now 48.1 on 27mm internal rims...but they are a little over inflated currently. I have a Parlee which is supposed to clear 700x40 and like your stiggy is more of a CX bike then one that was designed for 650b. I have almost no clearance ~1.5mm when running the 47mm Byways so I stay away from mud when running those.