Slant 6 to V8 cooling

Core support opening is 22 5/8" wide by 16 3/4" high. Rad core is 25" wide by 18" high. Rock Auto replacement rad for a 74 Duster with 318 motor and A/C. Don't have the hood on the car yet and height is my concern. Don't want to have to modify the rolled section on the bottom of the core support, but I might have to ( slightly ) to get the rad to sit low enough. I will bolt on the hood today and see where I am at. Drilling new mounting holes ( which I will have to do ) is not a problem. I am curious if Ma Mopar installed different core supports for the /6 verses the V8 motors? Thanks for everyone's input. Again, new to A body stuff.
On another note, Had to shorten the drive shaft 3.5" to accommodate a 727 verses the 904 slant trans. Also slight mods to column shift linkage. Linkage did clear the headers though.