You should be brought up on charges for doing that to that beauty.
I broke in my engine without a temp gauge. A professor of mine told me humans can only withstand temps of around 140-150F on the hand. My radiator didn't get much hotter than I could touch (on the metal), so the water wasn't much hotter than my thermostat rating (165 or so). You could do it that way, but make sure you're circulation system is flowing ok. Also, don't be afraid of a little smoke and fumes right after it warms up if you're dealing with a freshly panted/assembled engine. Just make sure they die down after about half an hour.
My 52 GMC 100 was painted the same original color. Nice truck, and good luck with your project.
