I suggest that the photo doesn't show an ash-framed ali-panneled body, but the wooden buck over which the panel beater (or wheeling machine operator) made and tested the ali panels of a prototype. The heaviness of the framing makes it look like a buck - a non-micro power plant would be required to drag that much timber around....
Back then using aluminium to make a prototype body from which a fibreglass mould could be taken was fairly usual - after all, the panel beating skills were readily available, albeit expensive even then.