Hi lemurfart,
Sorry, late in the game, but here's a couple of pics of the cardboard that goes between the seat and the rear seat brace, in and out of the car.
Also note, if you have'nt already, that the jacks are different for different years. I think the '70-'71s have a hook that grabs under the bumper whereas the '72-'74s? have a "finger" that goes into a slot in the bumper. The one pictured is from my '72. It may not be a Challenger specific jack so you might want to investigate if another car's jack can be used, but I'm not sure about this.