my 289 always stays pretty cool except when pulling the boat
as long as you can make shure your gauge is working the next thing i would take shot at would be pulling your thermostat and see how it heats then. you may be surprised that it will run cooler especially witht the smaller stock rad What temp thermostat do you have in it i could only find a 190 degree stat minumum and have been running it. on a hot day pushing the car it climbs to about 200 or a little better.
not much to lose but a gasket and a little coolant