Heater core isn’t fitting in Plymouth Duster

Take it back, and get one that fits correctly, you may have to upscale your search. In situation like this you may have to find a place for this type of specialty item, such as the two names that was mention earlier in the thread.
Maybe some day......... the parts stores will stop out sourcing parts from overseas after the warehouses get full of this crap. LOL But there will always be the ones, that will make it work, so the cycle continues.