Chevelle running hot

[70SS454]

I do have a fan shroud. The car was a Wyoming car it whole life until a few months ago. I have changed fluids but not the cooling fluids. It looks pretty clean and I have added several times because it dumps it over when it warms up. The car is pretty original. It belonged to my father-in-law who liked for things stay stock. I believe the timing is where it should be, not advanced and I think the radiator is fairly stock. The car runs beautifully. I guess I should start with the thermostat. Then move to installing an over flow tank like I did on my old jeep cj7 when I restored it. I really don't know much about this car yet.