A guy that worked for me years ago was big into demo derbys. I remember him saying that Imperials were banned because their frames were stiffer than regular New Yorkers.
He and his brother would swap the frames out hoping nobody would catch it. If they did get caught, they had a backup car to run with the regular frame. No idea to the truth, but it may explain why they are still around.
1966 and older Imperials are full frame cars. The 1967-73 unibody Imperials have a similar body tub to a Chrysler but the front sub frame is very different. The engine and suspension mount in a separate cradle than the frame and are alot beefier behind the front bumper. A lot of extra metal there.
