For your truck Im sure you can get a bigger capacity tank from somebody like JC Whitney or LMC Truck.
Ive got no clue with heavier trucks except I know diesels sell for stupid money even with miles on them. I know youll get much more for this one with some kind of bed on it than with nothing on it but the market is limited for it. If it was a normal F250 or F350 selling it wouldn’t be a problem but this you might sit on for a bit.
Shit, even half ton gas motors are going for gold prices these days with a new extended cab going for over $40k.