Jake,
No doubt this can be done. You would have to replace the entire rear clip from the door jambs back. I have now cut two C3 bodies up and put them back together. No big deal, just a lot of man hours and pre planning.
The trick is getting a 68-72 rear clip with good chrome, trim, and bumper supports for a reasonable price. Just the rear bumpers alone cost around $400 each!!!! So, this project could go over the deep end fast.
You might be better served to just sell the newer one and look for an older one in good condition, unless you have a line on a 68-73 that has a good rear clip dirt cheap. You really need the whole car, or at least the whole car from the jamb back to make a good quality swap.