Hello 2.3stangii,
 The 74-76 swap is not too bad except for the bumper mounts.
 The swap to the 71-73 style are a bit mor complicated as the 74-up core support and frame are bigger and will not allow the use ofd the front pan unless you cut the pan..
 From Pintony

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.

Better news. You don't need to change the fenders to go from 78 to 74-76 style. You just need to remove the rubber headlight surrounds from the fenders you have and get the 74-76 style flatter styled headlight bezels. I've done this the opposite way. I changed a 76 to a 77. It works.

You will need the grille. There are subtle differences between the grilles in the mid 70s, even tho they look similar.You may need to find a year specific brace/bracket to retain the grille that you buy. Not sure about that.

Hope this helps.

Well thats one part down, 76 Grill anyone?