Re: how to build/install a floor in alloy boat?
That might be a tough choice. Do you want a flat deck? Or a basic small flat surface.
Any decking will add weight and also make the boat a bit more tippy since you will be essentially changing the center of gravity.
Many people have made removable decking for the smaller boats with great success.
There are a few smaller projects in the Completed Project Section that reflect that and I would also look at a few of the Starcraft restorations for ideas.
Have you considered a nice non-slip coating directly on the hull?