Heater core isn’t fitting in Plymouth Duster

Who from? I use Rock Auto for a reference for part #'s and pics. If it was me I wouldn't be grinding the box to fit.
I also have a parts manual so I can Google original mopar part numbers.
I accidentally got the one without AC and took it back and they gave me this one and they said it was the right one for my car. I think I’m going to take a dremel to the box that the heater core goes into, so that the pipes fit in. I think that would work.