Very, very nice. FWIW, I think your estimate of 1973-74 is pretty good, based on the Velo Retro Cinelli timeline.
I agree that your choice of seatpost is appropriate. No way a Campy two-bolt will be long enough. I also doubt that a Nitto two-bolt (S-83? S-84?) would be long enough, although it would be closer. For your purposes, what you have is pretty much the best available option that I can think of.
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