73-76 doors are identical but are different from 77-?? doors. I found this out the hard way when i bought 77 doors for my 73 (the 77's were about $50 cheaper per door at the time). The main differences between them, at least that I'm aware of, is that the window tracks are different and your windows wont work in the 78 doors. If you want to run the 78 doors with the 76 guts you will have to swap them by welding the old tracks into the new doors. If you do this then your window cranks wont stick out far enough, you can only fix that by welding an extension on the crank or put power windows in. Your door panels will also not work. If you are going to swap a complete door with everything in it and on it from the 78 then you will be OK and everything will be fine, otherwise you are in for a little mess.