Here's a few things to consider:
It's never going to be 100% correct because the original is gone. So, what is the next best thing? THAT is the question.
Typical Choices:
* Casting date within a reasonable range before the cars SPD (usually 1-6 months for non Hemi engines), better yet, same block with an assembly date within 1 week-3 months before the SPD of your car.
* Warranty/Service replacement block, casting and assembly dates are less important since replacement would have taken after the car was built so the casting date could before the cars SPD, or many months or even years after the SPD.
So which one is better? Well, tough call.....The warranty engine is going to have stampings and small details that are somewhat different than an assembly line installed engine, the used origial is going to have the VIN# of another car stamped into it (So the "Ghost" of another cars DNA will forever haunt you!).