perhaps this has been linked:
http://650b.webs.com/conversions.htm
good measurements to keep in mind when checking out potential frames.

One key limiting factor is brake reach. If your bike had 27" tires originally (usually, what... pre 1985?) The reach to the rims on 650B gets to be waaaaaaaay long at times. A bike with 700C wheels originally is always easier. Tektro R556 are nice long reach brakes. Older bikes with classic fork crowns often offer more clearance. My 650B bikes run 38MM tires (Col de La Vie and Soma B-Lines) they are plenty cushy and roll really fast. The Hetres are cool but just too 'rich' for me at that price point.

I am taking delivery of a couple frames for my next project and am juiced to get started.