Ive seen a couple of headgaskets go even with these conversion kits, get a standard thermostat in, and just make sure you clean the limscale and corrosion out every two years in the thermostat housing use corrosion inhibitor and plenty of coolant changes, your head gasket will never go, when the thermostat housing gets blocked up with all the crap over the years of poor maintenance it cant open, mine was like this and many other people i have spoken to said the same, and if your headgasket does go, make sure you replace the gasket with a copper one, you really cant go wrong.
Welcome to the club John
regards
Chris