From what an engine machinist told me the other day, the early blocks are the ones to use as they have a higher nickel content in them. He also seemed to think the newer blocks tend to crack when they get hot due to lack of metal where it is needed.
Depending on the state you live in, a big advantage with using as old a block that you can find, is the pollution regs may go by the year of the block not the year you make the car. So, in a state like NJ with very tough pollution regs you would only have to meet the requirements for 1968/69 which weren't all that tough.
I would guess it is not original (69 was 1st yr. of W) to the car, from what I remember on my '70 351W block the D0XXX number is stamped onto the left (drivers) lower block flat ridge to the rear of the motor mount, just above the oil pan bolts. If you can jack the front corner of the car up and slide under with a good flashlight you should be able to see it, may need a rag for grease on it. The letter is the decade C=60's, D=70's, E=80's then the next # is the year, D5XXX = 75, etc. Jon
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