Need help getting started with kayak design

[syotr]

I am considering building a kayak with some unusual requirements:
Length: 9'
Beam: 21"
Planing style hull
I am thinking of a shape similar to the Prijon Athlete, except shorter, narrower and lighter.
Paddler weight: 145 lbs (No other gear will be carried)
The boat will be used for racing (down river and slalom) in class 3 whitewater.
Has anyone built something similar?
What construction methods would give me the lightest, fastest boat that would survive in this environment?
I have played with Hulls and Kayak Foundry software. Is there something better to use?
Any help would be greatly appreciated.