PMC,
Its a nice project. I have a simliar one in mind, w similar swingarm mods (tho I'm using an 82 mono swingarm & rear wheel), but styled along the lines of the early ATKs w their colour scheme.
Have you tho't of trying a plastic XR tank? An XR200 tank is a reasonble fit on my frame, but a bit short & needs the std seat foam reshaped at the front to look 'at home'. To my mind they're a much nicer shape than the std XL-S tank (which I think are ugly)
I don't have one to try but the plastic XR500 tank may fit even better cos its longer & would fill up the space better. They're 2.4 US Gal whereas XR200 is 2 US Gal, so neither are too bulky.
Its a bit hard to see much of the detail in the pics, so any chance you could post a nice side-on pic of reasonable size & good lighting of where the project is up to now pls?