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Do not know the average, mine is a July 1, 66 build date (VIN) and a mid May casting date for the heads, intake and block. I have heard as you got closer to the end of the model year casting dates (block, heads, intake) and build dates sometimes stretched.
How about cast and assembled on the same day. Saw that one time, and kept thinking no way. The explanation I got was that they were running three shifts, casting occurred on one shift, assembly occurred two shifts later. Still seems had to believe.
I had heard that, in the real world, any where from the same day up to about ten days. I don't know how the casting would be cool enough to machine properly in one day but I guess it was.
I have a block dated March 10 1959, my car build date is March 26th. I was going to have the block assy date stamped March 13th, looked it up on the "day" calender and it's a Friday the 13th.