For it to be done properly you need to know the correct shape and size to port them to or you can make it flow worse then before you started!! Experience counts for a lot here!!

"There are certain things that absolutely must be done to portshapes to get the CVH head to flow to its full potential. The area around the valve guide where the port bends is critical and this must be straightened out and shaped to blend nicely into the valve throat. You MUST remove the valve guides to get at this area so any head which just has a bit of polishing and no real constructive port work will not show much of an improvement"