What have you racers been getting away with for valve clash clearance lately? .050" used to be the minimum but I've seen high lift cams that provide less clearance. See the lightweight radiused lifters that I'm developng for Nortons (link below). This is an offshoot of the lighteight pistons (200 grams) that I developed a couple years ago. The higher RPM/lower vibration capabilities pointed the way to lighter valve train that could handle the higher RPMs.

see the lightweight radiused lifters at:
http://users.gotsky.com/jimschmidt/tapblocks.html

see the lightweight pistons and longer rods at:
http://users.gotsky.com/jimschmidt/nortonrods.html