Thanks. I had another Kish singlespeed 650B bike i bought from Kish but it did not fit me to well (demo bike, not custom) But i did really like the 650B wheelsize so when i was after a custom build i would like 650B again, i'am not that tall (177cm) so Jim (Kish) did not recommend going 29". For me 650B are perfect, the wheels don't look too big on the smaller frame, and the wheels don't feel heavy so the best from 26" and 29"!
Rolls very nice but still feels "light" and fast on singletrack.
And yes you can get the RR tires in 650B. On my bike mounted on super wide Velocity/Pacenti P35 rims.