I take it the car won't benefit from just relacement sections?
If you've never done them before, replacing an entire floor can be a daunting task. But it really isn't that difficult if you know what you are doing and go about it methodically and carefully. If you need an entire floor pan, my suggestion would be to buy a one piece pan that is already welded together and has the brace installed.
The biggest problem with those is you either need to have the windshield out of the car or you need to pick the body up to be able to set the pan on the frame and lower the body back down. The frame is the best jig for making sure the pan is aligned correctly before welding in the new floor. You also need to brace the body to make sure nothing moves with the floor removed. You can probably just run some square tubing from side to side at the upper rear part of the door openings and run some braces from the A-pillar (about belt line height) to the B-pillar to keep your door openings in spec. When you have the floor in place and ready to weld, recheck all of your door gaps and even your trunk gaps to make sure they are right before welding and locking everything in place. You can either do a measurement of the door opeings in several places before cutting out the floor and record them and recheck before welding. Or you can cut out the braces front to back and hang the doors and align them. You can even keep the doors on and weld the front to back braces inside the jam to absolutley make sure the body doesbn't move and see that the door fits visually.
Since these cars had shims under the body bolts to make things fit, you may need to tack the floor in several places and shim the body bolts to get your gaps right or keep them right before final welding.
Some days it's not worth chewing through the restraints.
1999 Silverado Z71 4X4 extra-cab short bed
1983 Malibu Fauxmad - tubbed
1978 El Camino Kustomized
1972 Monte Carlo
1957 210 handyman wagon
1957 Nomad sport wagon