Hi Tim,
the way i did my ali floor was to clamp the ali to the chassis then drilled all the holes, removed the ali then put the sticky stuff on
& with the help of a mate placed the ali back onto the chassis, using some old drill bits to hold the ali in place to the correct holes position(each corner so to speak), clamped it again then rivetted it.
might not be how everyone does it but it worked a treat for me
cheers
andy