How are you guys breaking it down to the exact day of build?
You can't. The date is called the Scheduled Production Date, it is that day they HOPED to build the car when the order was first entered into the system. For 1970 cars at Hamtramck, there appears to be a six week window, the cars were usually built within the three weeks before or the three weeks after the SPD.
Take a close look at your dates codes, lots of original cars have a part or three dated a few days to a week after the SPD.
Also, most folks mistakenly assume that the cars were built sequentially by the VIN number, this is not the case.