no heat in my 72 dart...AC car

I can't get any heat in my 72 dart. It originally had air conditioning. I bought a heater core....thinking it was plugged. I started to swap it out and it looks like the "box" that has the heater core in it probably needs to come out. Is that right?

The dart blows air, it just isn't hot. Is there any issue here since it used to be an AC car?