What is going to hold the body from falling to the floor when you remove the rockers? I see factory braces going from one rocker, over the frame, to the other rocker. If the rocker is removed, that brace will be doing nothing, it will simply be laying over the frame. What else is supporting the body? I know there are others, but what are they like, are they mounted to a rusty cowl? What will be holding up the body when you cut all that away?
It's hard to say being I don't know those cars at all, but looking at your photo it looks like your body needs to be supported, sort of like you are going to roll the frame out from underneath.
The lower piece of pipe going across the door jamb is the only one you have that is realistically going to do anything. That is IF the body is supported properly off the floor of the shop. If you plan on lifting the body and rotating it upside down or rolling it like a rolling pin across the shop, then sure cross braces aren needed. I don't think you plan on doing that.
To replace that floor and rockers you support the body at the cowl, quarters, and rear near the rear body panel all evenly. The frame should be able to be theoretically moved a little under the car, the body should be supported by the garage floor. You don't want the frame to move because of course it needs to have a few body bolts dropped thru the holes when you are doing the floor and rockers to keep everything in line.
There is also no need to cut it all away and remove everything at once. You can cut away the floor around a rocker and replace a rocker, trial fitting the door a few times during the process. This is one thing that the braces across the door opening can stop you from doing. Honestly, I don't give a crap about braces, I want to trial fit panels, THAT is there the rubber meets the road. Your welding point for that brace could bend a little (at least the way you have it welded). You could make some super monster braces going across that are well supported at each end that won't let it move, but I STILL want to trial fit panels, that is the bread and butter to doing this work, TRAIL FIT.
But you could do a rocker at a time and then do the floor. Think about it, you have those braces going across the floor from rocker to rocker, there are others that I can't see in the photo too. You pretend it isn't going to get a floor, you replace those rockers making everything FIT, making the body solid, THEN you replace the flimsy sheetmetal floor that is doing next to nothing structurally with that body.