Still overheating

Don't you have an overflow bottle installed yet? We have them on all our Darts. Also my sons 69 6 cyl had EVERYTHING replaced including 2 new radiators from parts store. They had micro cracks along seams. Also finally went back to the original 69 thermostat all brass. We were getiing some bad ones (New) Check them in pot of boiling water on stove. Good Luck!