I just went through this puzzle a year ago....
I have a 76 (16 GAL tank) and a newer bed with a fuel door. (Not sure what year bed as the truck was in pieces when I got it). The filler opening on the bed will tell you if it's a 79-83 or 84-87. LMC catalog is helpful with pictures. I have a 79-83 bed. The size of the filler hose also changed in those years. 73-81 had a smaller size than 82-87.
So, similar to your situation. I needed a fuel filler pipe for a 79-83 and a fuel filler hose for a 79-81. That would be correct if you have the 79-83 bed. If it's an 84-87 bed, you may have to adapt from the smaller diameter fill hose to fit the larger size filler pipe.