Headlamp Header Bar Installed 1973-1982. Beginning in 1973 the front headlamp header bar was installed directly on to the upper surround panel without bonding strip or the aluminum rivets that held it in place. These rivets on the prior production cars 1968-1972 were a constant source of galvanic corrosion which caused the rivets to expand and push upward on your headlamp panels. In 1973 this issue was solved by GM by not placing them in the header, it was simply glued in place.
If you look closely at the picture you’ll notice the red arrow, on the later front ends there were two lines on the bottom of the surround panel to help with the installation centering of the header bar.