For the most part, ignore the casting date. Go by the assembly date on the identification pad next to the dist. hole. Find a block with an assembly date relatively close to the scheduled build date for the car (and the correct model year), then check the casting date to confirm that it was cast before the assembly date (avoiding a shoddy restamp)

Example to make the assembly date over casting date a priority-

My 67 GTX came with what is believed to be the original block. SPD of the car was 9-13-66. assembly date on the cyl. block is 9-6. casting date on the block is 4-25-66.
Block is correct for the car even though it was cast almost 5 months before the car was assembled. Kind of an extreme case, but it does happen.