The valve you have circled is a port used to pressurise the coolant circuit, they are made the same dimensions as a tyre Schrader valve TR4, any machinist should be able to make one in alloy for welding to a header tank (as per your pic) or you could buy an aluminium valve from somewhere like Merlin motorsport or Demon Theives and mechanically couple it to the header tank on a flat surface.....

http://www.merlinmotorsport.co.uk/p1...duct_info.html