If you are completely sure that the bottom and front are structurally sound, you can most definitely just separate the pre-welded box. I believe it is only held together by spot welds. I bought the entire piece however.
If you decide to remove the entire thing, you'll understand how to put it back in as you remove it. After you remove the continuous weld in the wheel well, as well as the ALL of the spot welds, the box will still be in the car. You need to smash it out with a mallet. To get the torque box back in (bottom and front together, top goes on after), you also have to smash it into place with the mallet. Just keep hitting and it pretty much goes into place (or it did for me anyways!).
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Now the floorpans look BETTER than this 
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