The plot thickens, I took the valve cover off and the casting number on the top of the head was D 176. That would be April 17th, 1956. It makes sense to me, that the block and the head were cast around the same time, because according to the block's CON 4, D 166 is April 16 of 1956. I think they were assembled together at the factory from those dates. I hope.

A day without laughter is a day wasted- Charlie Chaplin
When wrestling a grizzly bear, you have to keep at it until the bear gets tired, not when you get tired.
1948 Chevy 2-Ton