My original 440 Assy date is 2 weeks before the SPD
The original 383 for my Super bee ASSY date is 1 day after the SPD on the tag ... both are original pieces ... it can be all over the place and the later in the musclecar era the longer the gap could be???
Casting date has to be before the ASSY date obviously , I have seen as close as 3 weeks myself, some have been closer. Hemis are all over the map and I know of a 71 6pk car that ASSY date was months before SPD.
It's never going to be original so get whatever you can that is before ...