The later shock mounted bumpers were just a band aid to get past a new standard. The actual structure of the cars wasn't redesigned.....they're still bolted to the same car.
Once again agree completely with 72's points above. They did nothing more than bolt on an extremely heavy bumper core, behind a standard chrome bumper, mounted on two springs to take small bumps without damage. Once the give of the springs is taken up the exact same rear structure as earlier cars takes on the impact. My '74 had the same treatment up front (heavy core/bumper cover) but mounted on standard brackets not shocks. There must have been a different standard for front impacts that year as latter cars had shocks front and back.