First welcome to the site
Secondly that's for providing the needed info from your car
Don't have allot of data points for 70 Cleveland engines and real build dates but will try and help a little
Since your car was build at Dearborn it's possible that some of the time spreads for engines was a little tighter IMHO than lets say San Jose just based on shipping time. Looking at other engines built during 1970 production at the same engine plant and their Dearborn completion date I found a spread between the real build date and the casting date of the engine of 20-34 days. Again very very small sample but hope it helps in some way.
Consider when comparing casting dates, you have casting, cooling, machining and then assembling the engine as well as transportation to the car assembly plant.
Hope this helps and that some others will offer some additional related (year and plant) dates for you