I don't know the right answer, but my guess is that the crew cab configuration probably affects the fuel tank location. I used a newer bed with fuel doors (82?) on my '75, and just pulled the filler tubes from the junkyard to get it to fit.
If you can get by for a while with the fillers in the wrong place, I would consider just buying a 40 gallon suburban tank and adding an in-bed filler. There are several internet articles on how to make the conversion. Its simpler in the long run than the dual tank setup with a six port valve, although you do lose space for the spare tire.
Bruce