97guns,
I used to have one on the '81. My father ( retired ) borrowed the truck one day and needed to put gas in it. He did not remember that it had a locking cap even with the key on the ring ( and with his poor eyesight did not let him see the key slot - pre cataract surgery ). When he returned the truck, he did ask me why the cap was so hard to get off. He did say that he just got a good grip with both hands and twisted the cap right off - no tools involved
. He managed to bend or break the locking lugs in the cap.
I vote for just trying to twist the cap off and see what happens.