The point of converting a 700c bike to 650b is that the smaller wheels allow wider tires, which generally have a higher profile. I converted a 90s Bianchi road bike a few years ago and went from 25mm tires to 38mm. The difference in the wheel/tire diameter was about 1/2 inch (smaller for the 650b) so the BB height was about 1/4 less. Not all that significant.