Floor

My advice would be that if you're going to use ali I would weld in some 40 x 3mm steel trip to take the seat mountings, from the outside of the chassis to the transmission tunnel. There have been SVA failures when the tester believes that aluminium isn't sufficiently strong.

If you're using ali, bond the floor in place with polyurethane adhesive/sealant (Sikaflex or similar) as well as riveting, it all helps the chassis strength.

Cheers

Chris