So, while looking around on the 'net, I noticed something. A guy found a bed on a '75, but it had an in cab fuel tank, so a filler neck could be a issue.
According to my book, on D models, the fuel tank left the cab in 74, and an optional fuel tank was in the cab. It doesn't say when the tank moved on W modesl, but in 76 the tank was raised between the frame rails...
On a side note, the dually was introduced in 74 also.