I have a 75 hood and a 78. They look identical to me. I'm fairly sure that the hoods interchange from 74-78. Earlier hoods look similar but may change around the front latch design. The bumper brackets on the 75 will interchange with the 78. They are a different style but use the same dimension mounting holes in the subframe. I would keep the 78 bumper/mounts because it is similar to the 74-76 but made of aluminum and is much lighter in weight.
Overall, I think that you could easily change your 78 into a 74-76 style, but it is more difficult to fo to 73 and earlier. Hope this helps.