Fwiw, I've done three 650b conversions, but all of them from smaller 26" MTB (559mm) to larger 584mm rims, mostly to avoid this very situation.
My '84 Univega 'Franken-Rover' was unusual in that it was designed for standard 559mm rims- but with caliper brakes (it also had a bit more BB drop than you'd expect for an MTB frame). I used Tektro R556s with Kool-Stop 'salmon' pads & had no issues with braking or stopping, even in very wet weather. The rims in this case were Velocity Synergy (machined sidewalls); the levers were Shimano 105/1055 SLR on Soma Sparrow mustache bars.
Ultimately I decided that using a 650b frame was better to begin with, rather than converting a frame designed for a different wheel size.
Last edited by DIMcyclist; 03-29-18 at 03:11 PM.