I love that bike, and can't wait to see it finished. I think this is what I would do for the seatpost:
I'd leave the amount of fork tube you have left. I wouldn't cut any more, but I would make one small vertical cut on the fork tube so that it can be clamped with a standard seat post frame tube clamp. Then I would make a series of drill holes as necessary to make it so that I could slide a seat post right through the fork tube and through the sturdiest part of the fork- the crown. (In other words, I would make it so that the seat post would slide right through where the front fender is normally screwed to the forks, underneath the crown). Then you could put a standard seat post clamp over the fork tube (with your seat post in place), get it tight, and you should have no problems, I would think... So long as the seat was staying low to reduce the leverage. This would be simple and would look good? Best of luck with the build, that is a sweet looking bike.