I helped a friend do this to a hardtop many years ago. Took a 70 and turned it into a 71. We had a huge stash of used parts to pick from as we both had parted out several cars, as well as him buying NOS and aftermarket parts for the car. If you start with a complete car, the cost to restore/rebuild the 71 will be a lot less than the cost to buy all the correct parts for the 71 and then restore them. If all you are looking for is the "look of the 71" and don't care if a lot of the parts are correct (70 vs. 71), then either car will cost the same to restore except for the purchase of the front sheet metal, grille, tail lights needed to convert the car...but that is only scratching the surface to make a 70 car into a 71.