1976 Dart Overheating and other issues

To be honest, that radiator top tank looks terrible. But looks can be decieving. Clean off all of the copper and iron corrosion residue. Then if there are any leaks when the system is hot (under pressure) you will see them, or at least the fresh evidence. It could be pinhole leaks, could be a cap or hose, could be a solder joint. Any loss of pressure holding ability will result in the temperature climbing above the normal range under extended use.