Well, there are many. I'll try to list what I know but you might want to PM Philip (that's his username).
Anyways
1. The dashboards from 75-76 are slanted, 77-79 are more of a boxy rectangle shape
2. If your car has A/C I do believe the A/C set up is different than that of a 77-79
3. Usually all interior parts interchange. Except the dash that I mentioned above.
4. ALL exterior parts interchange depending if you have a 2 door, 4 door, or hatchback. Grills from 1975 required certain brackets which are also the same as a SS/Rally grill which was also offered in 1975. Grills from 76-79 could vary (Concours, Custom, or SS/Rally). I had a Concours which we did a front end swap with and the bumper filler was different. Tail lights also interchange for the exception of the Concours lights that had 3 bars instead of 2.
I hope I put that in the least complicated way possible

if you need pictures let me know.