The head flange started out as 1/4" and due to some warpage during welding I surface ground it completely flat. on one end it is only 1/8"! but amazingly enough it worked fine and never really warped enough that it could not be bolted flat. Eventually after many 30psi (intake so what 60 in the header?) runs it did start leaking a little on the ends but nothing you could hear. As stated the copies will be 1/2" flanges.
Very close to the firewall with the E cover, Thermotec on the firewall recomended.