So you are saying your head currently has hardened valve seats installed already. If so you typically can't enlarge the bowl area thru the seat and cut it for a larger valve by too much. First you need to know what size valves the seats are machined for now. Its usually not good to start out with a head that has seats in it already, because of this limitation that is placed on these areas. It can be quite expensive to have the old seats removed and then recut the head for larger seats for bigger valves. I would probably look elsewhere for a better casting to start with, just to avoid having to spend extra bucks needlessly, to end up back at the same place basically.
