So, about 10cm of seatpost in the frame. That should be enough, and is probably about what the average insertion length was 20 years ago before going to the silly sloping top tubes. But, you do have a longer post to give it more leverage too.

What crank length are you using? 180mm cranks are generally available, and would drop the seat slightly over, say a 170mm crankset.

Oh, here are some comments about your KHS 747.
http://www.bikeforums.net/clydesdale...-seatpost.html

29.8 seatpost?

Anyway, that Skyscraper seatpost listed above is still available. Are you going with zero setback?