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I haven't a clue. If it were mine, I'd contact Lee Iacocca and ask him.
The 289 was introduced in September 1962 for the 1963 models, but in August 1964, changes were made.
The aluminum waterpump was replaced by a cast iron waterpump, and the block is different.
Early 289's (221/260's) have 5 flywheel housing mounting holes. Late 289's have 6.
The changes occured circa August 23, 1964.
Thanks Bill! I actually have a HiPo 289 cast on Oct 28, 1963. It's a five bolt block, standard bore, just waiting to get new internals... or a boat load of cash! Wait I have two five bolts... forgot the one in the car! Also have a cast steel Mickey Thompson five bolt scattershield on the Falcon.