Complete floor replacement advice

I would place the jack stands under the rear subframe just ahead of the spring hangers and on the front subframe near the torsion bar crossmember and then put a floor jack under the K frame just to keep the weight of the engine from pulling the front of the car down. While the car is jacked up take a look at the door gaps to make sure nothing is out of wack. Then tack weld the subframe connectors in first (assuming they are the type that dont pertrude through or weld directly to the floor pan) this will keep the car from sagging while the floor is out. Next step cut the floor out, then weld the subframe connctors in while the floor is out, they will be easier to weld the front half while the floor is out.
An air chisel works real good for busting spot welds (drill them out first) but you also run the risk of messing up the parts of the car that you need to save so be carefull. I would cut as much of the floor out that you can with a cut off wheel or sawzall and the deal with the rest of the pieces that are spotwelded to the rockers and crossmember, you dont get extra pionts for removing the old rotten floor in one big piece so cut it into as many little pieces as you need to.
If you have to cut the new floor pan I would slice it right down the hump, it will be easier to dress down the weld on the top and the transmission and driveshaft will hide the seam from the bottom, just tack weld the two pieces together once you get them into the car before welding them in so they dont move around from each other while fitting them to the car.