I see what you mean about the picture on p. 48, but I just clamped and glued my breasthook (adding thick fillets beneath it later). Note that on p. 53 you'll run two temporary screws into the breasthook to hold the rails while they dry. I'd be concerned about hitting any permanent screws when I drilled for the temporary ones.

   If you do decide to put in screws, I think one screw on each  side (along with your clamps) would do the job.

   I copied this idea from another builder...worked great

I used screws on the first outer rail, then removed them after gluing using a clamp on the first set of notches.  I did use one screw to hold the tip of the second outer rail while I set up the clamps.  Screw is hard to see, at end of clamp..it only goes into first rail and is out past where the rails will be cut off thus no screw holes visible in outer rails.

https://picasaweb.google.com/114717787929554738224/BuildingWoodenBoatCLCNortheasterDory?authkey=Gv1sRgCKCBzczm_aKs2wE#5977124604158769074