Is it still a unibody/subframe connector car or does it have a full chassis?
If it's unibody, I wouldn't cut out any of it unless you plan on welding in stock replacement parts because the floor is the frame from the front suframe back and any roll bar/cage is attached to it.
Why are you replacing the floor? Rust or weight? If it has a full chassis, and you plan on replacing the firewall, I'd do it first, then build the floor. It'll need aome sort of x-bracing under it for support and attachment.
Johnny