Cooling requirements are based on the horsepower output of the engine. It has little to do with engine displacement. So a radiator that cools a 225 at 60mph should cool a 318 at 60mph.
That's the real answer.

My '64Dog D100 318poly has a radiator support with a specific size opening for the radiator. If I wanted to use a larger radiator I'd have to cut. Catalog data for those early trucks is lacking. I looked up later trucks and it appears that the later six got a radiator with less width and an extended flange on one side to block off the unused radiator opening. Everything else got a radiator as wide as the opening.

Conclusion: Your truck will run as-is, unless you are pulling heavy loads, are going really fast, or are going up a long, steep incline.
If the radiator you have has a core narrower than the radiator opening, with a flange blocking out the rest, then you might want to consider a radiator that fills out the radiator support opening exactly.

R.