The partial VIN contains the serial number of the car it came in so unless you find that original block you will not have a number matching car (part of matching anyways). The best you can do is get a "period correct" motor (and tranny) whereby having correct cast numbers and casting dates of the engine components prior to the range of the cars build date range as stamped on the trim tag. That would, IMO, add minimal value to a cars resale value if you decide to sell it later. But for your own piece of mind you can go that route as many have done here.
Don't get hung up on numbers matching anyhow. Most cars have been raced and blocks and other drive line components damaged and swapped out for other motors and parts available at that time. I routinely look at resale of old cars through various venues and even though many that have real numbers matching do sell for more money, many that are not matching still sell for good money too depending on the quality of the car. Even some with openly disclosed re-stamped motors are selling for good money. Maybe not as much as a real original matching numbers car, but still good bucks. It all comes down to what people are willing to pay.
Life is short so just enjoy the car while you can!