[Lynn Eades]

Rob, All of that is of course incorparated in a modern cap. It might just require de riveting the cap part of an ordinary cap, and making a packer and gaskets to fill out the gap to the required spacing and setting up a jig with a pressure tester to check and adjust it.
Andrew, use a modern thermostat. Millions and millions of them are working perfectly everyday and if I recall correctly, Waxstats fail in the open position. The older style alchohol type bellow stats fail in the closed position. (these can cook an engine) As I said in a previous post; Thermostats are there for a reason. They often get the blame for cooling system problems, which are usually the result of other areas of neglect.