I have a '68, the chassis and engine is definitely the same as the following years.
Also, I firmly believe the '71-'73 "hardtop" body is a different tooling than the '74-'76 "colonnade" top body. Way too many small changes to things between them that did not change on the 1/1 cars such as the shapes of wheel openings, trim around the windshields, the long body crease that runs the length of the car, etc. Not to mention tool marks on the inside of the bodys.