one to two weeks is normal for most high production motors. in the Corvette world NCRS allows up to six months. and in some rare cases of odd ball suffix code stamped blocks it can be. at that time there was no first in first out rule
In my experience it is the opposite, at least for 66 and 67. Special order low-production optional engines almost always have the tight time frames discussed above. "Standard" engines (in other words, a 396/325 for an SS Chevelle) have the tight time frames more often than not, but they can have VERY wide date gaps, due to the lack of FIFO inventory control by the car assembly plants. If they projected building yay so many tens of thousands of 396/325 cars (including a large percentage of dealer spec'd inventory cars not ordered by a particular customer), then they did not wait for each "car order" to come in before assembling and shipping large batches of those engines. And particularly at the very beginning of the model year, a large beginning inventory of those engines was sent to each plant. As a result, when new shipments came in they often got put in front of what was still left of the prior shipments, and those newer engines got installed first.
So there is not really any "bright-line" rule, especially when it comes to standard engines. I have seen late August 1966 assembled 396/325 engines (with original stamps) in March and later built 67 Chevelles from EVERY plant except Framingham (sample pool from there is so small it is difficult to even find Framingham cars for reference, much less original Framingham cars with unmolested original drivetrains).
On the other hand, low production optional engines (like a 67 L78 for example) always have tight engine-to-car date spans, because those engines were
built when those cars were ordered (whether by a particular customer, or by a dealership for inventory).
As to the original topic of casting dates, Tonawanda also did not use FIFO inventory control on cast parts either. So a block (or batch of blocks) could get "passed" by newer blocks for a long period of time before inventory was drawn down to the point of using those older blocks. The only firm exception to that was at model year change over, when blocks that would become obsolete the following year were pulled and used before a newer casting number block would go into effect for the next year. And even then you sometimes see a 67 406 block in a very very late 66 Chevelle, even though there are other 961 blocks with later assembly dates. And likewise for 68 323 blocks in very very late 67 Chevelles.
-65 Z16 Survivor (Red/black/white int.) (Survivor Z16 Picture album
-65 Z16 drag car, unrestored (Red/Red)
-65 SS Convertible (Red)
-65 300 2-dr Wagon, Modified (Tuxedo Black)
-66 SS L78 Coupe (Marina Blue), unrestored
-66 SS L34/M20 Survivor Coupe (Aztec Bronze) - 'Vintage Legend' Certified (Survivor 66 Picture Album
-66 SS Convertible (Marina Blue)- "LOADED"
-67 SS L78 Survivor Coupe (Grenada Gold)
-67 SS L34/M21 Survivor Coupe (Tahoe Turquoise)
-67 SS L34/M20 Convertible (Bolero Red)
-67 ElCamino L35/M20 (Butternut Yellow)
-67 Concours Station Wagon (Mountain Green)
-67 Camaro SS350 Survivor (Grenada Gold)- 'Vintage Legend' Certified by the American Camaro Association