it was a thought for what can he do without spending $$ and aim towards cleaning the muck out of his fuel system in a pinch. chances are the fuel tank in his other truck doesn't have any rust if it still has good gas in it and it's not rotten gas.There are companies that have tank coatings. Not sure how to use them or how they work. If you do have rust in the tank, it might be worth looking into.
I would find another tank to use rather than mess with the gas tank.
The junk yard might work for that.
a new diesel tank would cost in the neighborhood of $200 including a new sending unit. from RA. I think getting one coated isn't worth the cost on these trucks.