Don't know about 64, but I have a 67 slant six radiator in my 63 Valiant and with a little modification it fits perfectly.
The 67 radiator opening is a good bit wider than the 63 opening, to be able to handle the V8s I suppose, so while the 67 slant six radiator itself is no wider than the 63 version, it has that large filler bracket on the passenger side that the 63 doesn't have. All I had to do is take a cutoff wheel and cut away most of the filler bracket on the passenger side of the 67 radiator, then drill two new mounting holes on that side. It would take a close inspection to notice that it's a later radiator, as it looks and fits just like the original.
