Do you have a 26 x 1 3/8 wheelset from an old cruiser around? That give you an approximation of how much clearance you'll end up with 650b, the different is 6mm between the two sizes (590mm vs 584mm). I imagine you'll need long-reach brakes for this to work as well, since the difference is radius ~19mm for the change you wanna do (if I did the maths right). I can't comment on 650b rims but there are more and more being sold these days, velo orange and long leaf bicycles seem to have a good selection of stuff, but tektro brakes are available everywhere and they still make dia-compe centre pulls that should give the needed reach too... lots of great stuff has been posted about 650b conversion here too.