Slant HD radiator for 318??

That's a really ingenius idea lol, I might try that out. It's only a 1/4" difference so it should hold pretty well.

My other thought was since I plan on selling it I wonder if having a slant radiator might turn someone off since it would kind of be incorrect for the car.

I did find a couple aftermarket rads that would work.
This one seems like it might be too wide for mounting holes....
https://www.summitracing.com/parts/sgt-cu1707

This one I am just unsure about because Ebay....
3 ROWS/CORES ALUMINUM RADIATOR FIT FOR 1970 1971 1972 Dodge Dart 5.2L V8 | eBay