Not entirely true either.
They are interchangeable but if your restoring and need "correct" frame rails, there are differences between 63-66 and 67-74.
The early ones only have one alignment hole at the bottom back of the rail (the later ones have two plus a slot between them)
The later frame rails also have an access plate that is spot welded in separately on top at the front where the bumper bolts on. (the early ones are one piece top plate).
