Putting 81-87 fuel doors in a 78 short bed?

[rs74]

How hard would it be to take a 78 short fleetside bed with the round access fuel caps and put in the 81-87 square fuel doors. I am talking to a guy who has a nice 78 short bed for sale but I have an 85 and want to keep the square fuel doors. Do the 78 beds have the fuel caps in the same place as the newer beds. Thinking maybe cutting out the doors and putting them in the older bed. Any differences other than the fuel doors.