71 Header panel is unique due to several of the welded in brackets. Below are the differences, unfortunately I don't have pictures to illustrate the statements.
1. 71 center support is different form 72-4 in that there is a support welded to it to strengthen for mounting the horns on the later ones. You can't simply remove this support to make a 71 because the strengthing rib in the main part is different on both as well.
2. The 72-4 have an extra support near the middle that the 71's don't have.
3. The end supports where the header bolts into the fender are different as well. The 72-4 are wider and the "keyhole" for the grile mounting plastic nut is rotated 180 degrees from the 71.
Items 1 and 2 above should not affect using a 72-4 in a 71 but I would imagine that #3 may give you a problem when you go to put the grile in.
Another point of interest is that I beleive the outer sheetmetal 70-74 is the same.
I recently modifided and NOS 72-4 header into a 71 using my old brackets. Not a hard project just time consuming since I am not an expereinced body man just a hobbyist.
Mike Afflerbach