That looks pretty solid actually.
To get to those bolts on part E you will need to remove the torsion box end cap and then use an air tool with a deep socket to get to those bolts from the rear of the torsion box.
Or (if you were going to rebuild that area anyway) you could just grind the bolts off that are on the inner rocker and then put new bolts/nuts back on when you get done. However, that piece [E] is also welded to the floor step up area so just taking those bolts out will not allow you to just remove piece E.
Here is a pic from
www.gmcpauls.com site that shows these pieces as well. Note that in this pic the captive nuts are welded to the inside of the inner rocker/cab support piece [E] and not the stock way as noted in your pictures.
Note the spot welds holding it to the floor.