old thread I know, but curious if a dave scott conversion occurred. I am looking at a dave scott centurion to buy, and am thinking about 650b. I might just use pclamps for fenders if necessary and leave it be, but.... As for a rack, you could always put a decailleur thing on the handlebars and use a handlebar bag instead of rear rack. Such a bike might not handle well with extra weight on the back anyway.
I realize there may some headscratching as to why one would want to do this, but trust me, if you have a limited budget, getting the modern equilivant of a tange 1 bike or similar would cost a fortune. Even with the cost of repainting and adding braze ons a conversion costs less. Using an old loved road bike or second hand vintage is the only option really. If you are short like me, it can be hell trying to find a high end bike small enough, and a 650b conversion is the way to go. Modern 650B framesets are still limited to boutique, custom and expensive. Conversions cost much less, but you would have to settle for 650b x 32 or 38 instead of glorious 42 hetres. If you read bike blogs etc, you read about people who have endless resources buy the latest fast 650b trail capable road bikes with hetres blah blah. It can get frustrating, you want to try it within your budget.
I would not go 650a, I have a 650a wheeled bike and the tires options are limited to comfort, city riding, touring and recreational. The only goodie is the panaracer 650ax38 col de la vie which is even too wide for road conversions.
From experience hybrids are horrible. They are not designed for riding long distances, or riding fast, racy etc. They are designed to be recreational, meaning mostly sitting in garages except for occasional short ride.