As with just about any full frame car made the frame is a square cross section, i.e. fully boxed. The result is a car that protects its driver and passengers in a collision better than just about anything on the road under 5K pounds.
Whats so important about this to you anyway?
Most trucks aren't fully boxed. Doing so is more of a gimmick to sell pickups anyway. You'll find the heaviest duty trucks, which are built to tow/haul the most and be the most flexible in terms of what may be attached in leu of the bed still use C channel frame.