Here are some photos of Peter Fouché's Tmax with Hossack-type FFE to give you a better idea of how the front end works:
Peter Fouché's T-max based project | FF Web
The AN400 seems a better starting place for a mileage project than the 650 but I'm well aware that sometimes projects start with what you already have in the garage. Some projects do remind me of the old story about someone being asked for directions and replying "if I were you I wouldn't start from here".
There is a shortage of powertrains that work well with a low seat and an FF riding position that would make Vetter-type aeros (to try and stay on the thread topic a bit) possible. The scooters have odd-sized (to me) wheels/tires and the rear drive on them makes it very difficult to change wheels. The new Honda 700 twin is much closer but still not ideal, and while it gets good mileage the engine capacity is still overly large for a single rider aiming at higher mileage. It is also new enough that used examples are still pretty high on the price.
cheers,
Michael