replace the whole floor or just leave it alone

[Schonyenko2]

As said before, a lot depends on either your ability, or if you hire it done.Also be careful of where you send it if you hire it done. I've heard some horror stories latley about guys sending their cars to regular dealer body shops with less than desireable outcomes. There's a difference between a collision shop, and a restoration shop. Kinda keep that in mind.
The dii full floor as Stephano said stops at the toe boards, but does run past, and includes the rear spring perch/torque box area. If your floors a disaster area, that's the way to go. You do not have to put it up from the bottom if the steering column is out, and doors off. You can, with help put, it through the side.
I'd suggest you weld in braces from the door jamb, to the rear firewall area to insure theres no flex.
FWIW, that's my 2 cents.