that sissy bar looks like it could buckle right where the holes are drilled, it's already got a slight bend to it...
i'd find the perfect seat height then just weld the post in place (to the frame) and canabalize longer chain stays from another bike and weld them directly to the post... that's sort of the design of cheap, older step-through road bikes.
Looks cool though, looking at that makes me think of this website http://www.vintagebmx.com/