I've got a 500 in my -68 SDV, it´s in 188F when I drive and in 212F when I've been standing still on idle in 5-10 minutes. When I drive again it will be in 203F. And after awhile it's about 190-195F, is it ok? How high would it be without damage? I've got a new 180F therm and a new radiator cap, but I'm missing the radiator shroud. Just little worried for overheating at idle, when the car goes very slow. It's not boiling jet, but I can hear a "buzzering" sound somewhere when it's hot. The vacuum advance is ok.

Sounds good, really. The service manual for the '68 caddy says the thermostat was designed to open at 217 degrees coolant temperature. I wouldn't worry unless the temperature continues to rise after you start accelerating.