edh, Here is how I did the seat on my Gasser.
The seat post is part of an old Schwinn handlebar. I welded a small plate on the end so the seat clamp could not slip off. Also I welded steel flat bar over the chainstays to mount the sissy bar to.
Hope this helps you.
![Seat%20detail_zpsmxl1qvbw.jpg](https://proxy.imagearchive.com/852/8522edfc21ba2ea6e160e3892222ee39.jpg)
The seat post is part of an old Schwinn handlebar. I welded a small plate on the end so the seat clamp could not slip off. Also I welded steel flat bar over the chainstays to mount the sissy bar to.
Hope this helps you.