My 75 Highboy has a gas tank on the bed also along with the in cab gas tank. The bed is not swapped all factory. But while the gas door is factory the gas tank that sits there is definitely not

 

My 75 also has the fuel door opening in the driver's side near the rear of the bed. No tank or anything underneath, and no trim or anything at the opening. Been wondering whats up with that myself.

 

'73-'76 highboys had the round block off plate, or 'plug' if you will.

'77 highboys had the square fuel door, though it too led to nowhere, it was just a 'dummy' door.

 

After 60 Minutes did the exploding cab bit in 75 or 76 the big push was on to put the tank outside. They had the bed designed and in production before they got approval(from the DOT I believe) for the tank design and to locate it in the back.
As was standard policy with Ford, they used whatever bed they had.
My 77 is actually post a 77.5 Custom and came stock with the dual tanks/toolbox/400/4spd/married transfer case and sits comfortably closer to the ground than the pre 77.5. (Yes I am getting old)
I just found the right bed for it after about two years of searching. I now consider that to bethe Holy Grail of Ford pickup beds.