1974 340?

Hey y'all. I just cleaned up what I thought was a 318 that I pulled from my 67 Dart. It is stamped 340 in 2 different spots with a date code Jan 1974. Since the engine wasn't offered, as far as I know, in 1974 I'm at a loss. Only thought I have is that it could possibly be a replacement block they made in early 74 but that is only a guess. Any info would be greatly appreciated, thanks in advance.

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