Heater core isn’t fitting in Plymouth Duster

In post #1 you say your car is, "1976 Plymouth Duster without AC"

In post #4 you appear to say you got the one without AC, and went back and got the one for an AC car.

If you do have a car without an AC, then the one in the pic is wrong for your application.

These should be your part numbers if for an AC unit, Chrysler: 3502462, 3502765, 3780551