you might end up tearing the toe board up a little removing the old torque box, but if you are methodical, you probably can break open the repro torque box and only use the pieces you need instead of the whole thing.
The TB's have an extra wide flange that extends between the toe board and the front frame rail and between the cowl side panel and the toe board. So "Complete" removal and replacement will require removal/replacement of toe board also. However, if you cut the TB out one section at a time and trim the frame rail flange on the replacement TB, you will only need to cut a small slit in the toe board next to the cowl side panel. Then you can slide the TB up and into position.