In that Dayclona picture, maybe the challenger board is just sitting up straighter than the gran coupe board so that it appears longer.
Maybe, I had a spare GC board & a SE board that I stored for years, one stacked on the other, I think they were the same length, the SE used a rear window trim ring that was probably 3" wide that got covered with the same fabric as the headliner, that may be what what your car was missing causing the headliner to seem short...