There are a few very good threads on swapping in a F150 tank. It tucks in between the frame rails behind the rear axle nicely. However, I think it's only 20gal and the ones I've seen all do it to replace the stock tank which is prone to trail damage. So it's not for range.
The overlanders all seem to be using jerry cans and / or rotopax for extended range.
All that said, the absolute best way to extend range is to make sure your engine is running tip top, your tires are properly inflated for the terrain you're on, and to simply ease off the skinny pedal. I have a huge problem with that last one.
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