The early 70's didn't have crumple zones and had smaller bolts like was said.

If you are going for OEM correct then it matters otherwise they will all interchange and look/fit fine.

The only other difference is the 71-74 hoods had pull rods for the second latch, the 70's had a tab. There is a raised spot on the underside of the hood for the pull rod mount which uses 2 screws and a bracket. If your installing on a 70 it's no problem. If you have a 70 hood on a 71-74 take care when mounting the rod support bracket so it doesn't dent the hood from underside.