Let me elaborate to add clarity: as Cavemanmoron's picture shows, the 64-and-older cars have a narrow opening (19") in the sheetmetal, which defines the width limit of the radiator if you want to have the full airflow available. The 65-and-newer cars have a wider opening, for more airflow that the V8 needed.
When a 65 car was built with a slant six and a little 19" radiator, the radiator support pieces were really wide to make up the difference. (Wish I had a pic handy.)