That is usually where the rear spring hanger starts to rust from. Check and make sure it has not effected the spring hanger, (from under the jeep). If it has, the spring hanger will have to be replaced or a new one made.
As for the floor, clean it up with a wire wheel/ brush, lay your new metal on and weld all the way around.
This is a structural part of the body, it would be better to remove the rusty area and replace with a new piece of metal, but it can be patched, and welding is a must. This is fairly easy to fix, just time consuming. The most important part is to check the status of that rear spring hanger.