I've patched worse looking floors than yours, using sections cut from another car. Now is the time to paint inside those crossmembers and such that get covered once the floor is on. I used that black rust mort like stuff only it comes from Rustoleum. Not really paint, more like black phosphate treatment. I'd also consider tying the frames together if you're going to put some horsepower to the ground. Whatever you do, you might tack in some crossbracing temporarily from door sill to door sill while the floor is out to keep the integrity of the body. All that's holding the car from sagging in the middle are the rocker panels, and if they are iffy your car might not be square or level come time to put the floor back in it. Whatever you do I would not jack the car up by a corner. The floor acts as a huge part of what holds a unibody together.
