Longhorn 72 and Longhorn 68 question!!

[OregonC20]

After dealing with a couple of dishonest buyers on Ebay, I have decided to keep the 68 50th Longhorn. I also have a 72 Longhorn that I would like to use to get parts for the 68. Now, the 68 can use better fenders, a better bed and doors, which the 72 has.
Now my questions are: what parts of the 72 can I use for the 68? and If I replace the items needed on the 68 from the 72 or any other year, when do I stop calling the 68 a 50th anniversary? I want to keep the 68 a 50th anniversary. The 68 is running and the 72 is not. I hope that all of these makes sense. I am going about this!