Found my dads original 1971 duster!

A shroud will make a difference, as long as it is the correct one and the fan is spaced properly, it should be about 1/2 way into the shroud. Did you get a clutch fan or still have the mechanical one? I wonder about the thermostat as well, but at this point I don't think you would still have an air lock in the system, but anything is possible. Also did you replace the radiator cap?