Spark plug question

I have a simple question regarding spark plugs. Autolite 66 was listed for my '74 318. But it's not stock, it's mildly built. However the 360 has autolite 65 listed. Is there a real difference in the plugs? Should I have gone with the 65's instead? I did a search and it seems just about everyone running autolites in a small block use the 65 plug. Any info is appreciated.
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