A friend has two heads for sale.
First is an excellent casting supplied by NS Engineering who pressure tested and fitted new seats. Requires new guides. Full thickness.
Second is a proven undamaged casting having covered 4000 miles since modification which inc new big valve seats and colisbro bronze guides, Kent 450 cams, complete Iskenderian race valve train. Sprint thickness.
Photo below was taken pre-installation.
if anyone is interested, I can affect an introduction.