If the valve seats were correctly cut then there should be no need to pressure test , unless of course you are wanting to pressure test the fresh head with a worn one to determine or compare the difference in loss if any , if that is the case then i would say do it like they do with automotive heads and make a plate out of thick lexan (1 inch or so thick) and stud it or use nut/bolts , use either with large washers on the plate side with O-Rings to assure sealing thru the threads/holes , so you can tighten it down to the head and assure the combustion chamber is sealed (you could if you find it leaks , add a head gasket , and/or you could also use a 1/4" or thicker Steel plate , but the Lexan would be easier to drill and cut down to size) , then thru the spark plug hole , insert/screw in the adapter hose from a Leak Down Tester (you will need a Leak Down Tester to be accurate) , and use the Leak Down Tester as your guide , as they are made to test from the plug hole , and find any leakage , you can then accurately monitor any pressure loss either immediately or over time (24 hr , etc) from the valve seats ....
I use my Snap-On Leak Down Tester at about 100 P.S.I. on my bikes with the head on the bike/completely assembled and ready to run , as i can then tell if there is any ring or gasket leakage as well as valve sealing issues ...... same way it is used in automotive applications , but if your wanting to "bench test" then you will need a plate of some sort to seal the combustion chamber
