Funnily enough I was watching American hotrod on Quest earlier and they were building a car totally out of ali, even the axle tubes and diff housing were custom made in ali and it did make me think about a roadster chassis in ali.....................BUT:
Ali has totally different strength characteristics to steel so the existing roadster chassis design is unlikely to "port" over to ali fabrication. Ali is also a bitch to fab as heat distortion is a lot worse than with steel. you'd need to research exactly what grade of ali to use as many of them can easily fracture if subjected to repeated stresses and strains.
Finally, why bother? The general consensus is that the Haynes roadster comes in at just shy of half a tonne kerb weight, the chassis probably accounts for around a fifth of that weight, making it from ali will maybe cut the chassis weight by 50% (you'll almost certainly have to use heavier gauge material to compensate for the weaker material) so you might save 50-60KGs (purely guess work) and end up with a chassis that will not last as long as a steel one.
Just my humble opinion,
D.