344,
I have a challenger. I honestly don't know how your body guy pulled it off. The math simply doesn't work without cutting something. I'm fortunate to have a Plasma, so I was able to quickly 'trim' where required.
Larry,
I'm not sure how this guy did that either... The rear floor pan has the bumps in it that go over top of the rear shocks. Other than that it is pretty flat and could probably slide under the tail panel, but those bumps make it impossible (at least on my Challenger).
Jon