I need some thoughts as how to proceed

You will need to support the car carefully, remove the driveline and complete interior, and remove the K frame and torsion bars. You'll have to brace the unibody front to back and side to side. The floors are in 4 pieces from the factory - there are hundreds of spot welds, and the frame rail has other juntion points with various bits and main support members. I would remove the floor(s) first so you can see the extent of the rust. Generally the floors are not the only parts, and if the frame rail(s) went bad, there's other stuff that conncts to them that is bad too. It's not a small job, and the "might as wells" will have you doing a complete resto unless you toss all the old stuff back on it.