The interior is slightly different. Depending on how "correct" you want to be will determine how far you go. I believe the doors will have different handle mounting and inner structure. The rear seat area and belts could be different as well. But the outer structure is bolt on, excluding the tail panel. The bumpers will need to be changed as well, 73 and later are bigger with rubber pads. I don't think the bumpers had shocks until 74. I could be wrong on that though. Overall, it is fairly easy.
