1974 dodge dart overheating problem

A few things come to mind, you may have flushed the raditor but are you sure all the passes are clear? The raditor cap may be defective? A bad head gasket will cause overheating. Did you leave a air gap in the raditor when you filled it up. When when water heats it expands so you need to leave the raditor less some water for expansion....

Everything joemorn said and you have to burp the system, run motor with radiator cap off until the thermostat opens up and circulates the coolant adding coolant until full. Fluid may be pushed out of the radiator, but that's the air coming threw, add fluid when the level drops until full.