Are the quarter extensions different or does the bumper set where the old taillights should? Obviously you have to add the bottom valance metal where the old bumper was.
Quarter panel end caps are different. Also, left and right tail panels, center tail panel, bumper and brackets, and yeah rear valance all different on a 74 up.
Biggest pain was tweaking the rear caps to align them with the rear quarters. Especially since I had one broken stud. Had to drill and tap that for a screw.