What is the attraction of converting a bike to 650B wheel size? If it started life as a 700c or 27" wheel bike then the conversion requires installing much longer reach brakes and the bottom bracket height is lowered by about .75" or a bit more unless the tire width fitted is greatly increased.

I can see possibly converting 26 x 1 1/4 (597mm BSD) and 26 x 1 3/8 E.A.3 (590 BSD) bikes to 650B for better tire selection and availability though I understand 650A tires are even more readily available.