Thanks everybody for the input. I have talked to four different people that say they have 1966 motors but have the 2465315 casting number heads. I just don't understand it. Every thing I read says they are not compatible.
Until a casting is machined, no one knows where it's going. The fact that 315 casting have turned up in 66 engines, even though they are supposed to be 64-65 heads, just means there were alot of raw castings waiting to be machined; if you will, it's a documented factory mistake. It feeds into the saying of "don't buy a car built on a Friday, or a Monday".