I'm currently swapping my 74s tail panel for a 70. It is not an easy undertaking and will require a donor car. They do not make reproduction tail panels. You'll have to cut the rear off the donor car and weld it to the 74. Also, you'll need to expand the trunk floor of the 74 to connect to the 70/71/72/73 tailpanel. About 1.5 to 2 inches roughly.
You'll also need the quarter panel extensions from whatever year car you cut from as well as where they connect to the quarters as the holes for the extensions are in different places on a 74.
So far, I've got a little over 12 hrs labor just in cutting and trimming to get mine to look and fit right. And its not even in fully yet as i need to finish the trunk floor
My advice, sell the 74 and buy you the year dart you truly like.