Yes, adding a few extras layers of sponge padding is fairly typical. The new covers are designed around a new cushion, not a 40 year old squashed one. The previous comments were about putting them over existing seat covers, which I'm sure wasn't the intended method asked but many have. If you look at other Firebirds with recovered seats, likely by the owner, often there's a few wrinkles and heavy creases. Usually, one each side in the base and similar towards the top shoulder area. This is where the covers are too big for the filling, the inner cushioning isn't being compressed or the cover not being pulled down tight enough when fastened. Same result.