I've posted a few photos of it elsewhere, but here it is again, my 650b Trek 820:



Older MTBs (especially from the 80s, before the frame geometry became more aggressive, more like enduro motorcycles) are pretty good frames to convert since they have very long wheelbases (+ 100cm) whose proportions & slack geometry will overcome the routine 45mm BB drop to maintain a stable ride and handling even with the larger wheel size. I've tried a 650b conversion on a shorter-framed MTB (97cm) and it was proportionally too high-centered for city riding; the handling became tricky, awkward in the turns.

Bear in mind that most 700c road frames have deeper BB drops and work better with 650b wheels; even with a compact frame, the ride is stable due to the bike's overall lower center of gravity.

My current winter/ summer project is an (apparently) native 650b hybrid frame, a Panasonic from the early '80s- 26" frame w/ 60mm BB drop & 107cm wheelbase; 366mm fork w/ low-mounted canti bosses: smells like 650b to me.