I am thinking of replacing the hardboard headliner on my 72 Demon with a 71 headliner. I have done several headliners in the past but have never had a hardboard one. I know I will need bows and headliner with bow retainers but what else is needed? I also see there are 4 bow and 6 bow available. What determined if the car got 4 or 6 bow headliners?