To: supercuda, the stamp on the Engine ID pad only tells you how the engine was built and for which model year. So your '64 left the factory with a leftover '63 engine.
Chrysler had terrible inventory control and they had a habit of finding a bunch of blocks or heads or engines that had been "hiding" in the corner of a warehouse somewhere and putting them in the next car down the line. Remember, these are the guys that built engines with one or more rod or main journals, piston or lifter with repair sizes.
They salvaged everything they could.

R.