I use Evans NPG coolant with a zero pressure cap.
my spare engine is an old Comp engine that has some heads that leak a little, not unlike some SS heads.
This stuff works great and is race legal as it is not an ethelyne glycole base and cleans righ up.
It boils at 360 deg F at no pressure and has a very low expansion rate.
not cheap, but it works great and can be reused for several racing seasons.
http://www.evanscooling.com/products/coolants/npg-2/