Looking good. Without wishing to cause offense, when I look at your latest pics, may I offer my two cents worth?
With the angles you're working, an old school (hate that term, lol) straight post to the back side of the wishbone might look good. You could go with a Troxel-style seat then. You'll need some leg room after all.
Support wise, it doesn't look like you need a straight support across the frame. I'd be looking at a peanut tank with a stock Harley style tunnel mount on grommets, and back tab bolt.
Just my comments on the look- heck you're doing an excellent job on this and I find it interesting and motivating to watch. Thanks for the updates. :thumbsup: