'64 models are a bit more involved than the later cars. I have 2 '64 cars here, both came with V8's, & they have 2 different sized radiator openings, neither of which is as wide as the opening on the '65 & later cars. The biggest problem is that the core support is offset toward the front just to the passenger side of the radiator opening. If you cut the support for a wider radiator you're going to have to fab some type of bracket or something to make up for the offset so the radiator will sit square across the front of the car & air won't try to go around the radiator instead of through it.
The way I plan to take care of this on my car is to swap in the right hand section of a '66 core support that I've purchased. This allows a standard '65 or '66 radiator to bolt into the car. I think Moparlee did the same thing to his '64 Valiant, but he might have swapped the entire core support. It could be done either way. Not sure which is less work. I have the entire '66 core support I could swap but doing that would change the vehicle number stamped on the front of the car.
